diff --git a/src/common/ThreadStart.cpp b/src/common/ThreadStart.cpp
index 309e3da45b..574bc0e040 100644
--- a/src/common/ThreadStart.cpp
+++ b/src/common/ThreadStart.cpp
@@ -38,6 +38,7 @@
 #ifdef WIN_NT
 #include <process.h>
 #include <windows.h>
+#include "../common/dllinst.h"
 #endif
 
 #ifdef HAVE_UNISTD_H
@@ -319,7 +320,12 @@ void Thread::start(ThreadEntryPoint* routine, void* arg, int priority_arg, Handl
 
 void Thread::waitForCompletion(Handle& handle)
 {
-	WaitForSingleObject(handle, 500);
+	// When current DLL is unloading, OS loader holds loader lock. When thread is
+	// exiting, OS notifies every DLL about it, and acquires loader lock. In such
+	// scenario waiting on thread handle will never succeed.
+	if (!Firebird::dDllUnloadTID) {
+		WaitForSingleObject(handle, 500);
+	}
 	CloseHandle(handle);
 	handle = 0;
 }
diff --git a/src/common/dllinst.cpp b/src/common/dllinst.cpp
index b21121722f..aafd10a749 100644
--- a/src/common/dllinst.cpp
+++ b/src/common/dllinst.cpp
@@ -35,6 +35,7 @@ namespace Firebird {
 
 HINSTANCE hDllInst = 0;
 bool bDllProcessExiting = false;
+DWORD dDllUnloadTID = 0;
 
 } // namespace
 
diff --git a/src/common/dllinst.h b/src/common/dllinst.h
index 02f9d1aa9d..678b6b47c4 100644
--- a/src/common/dllinst.h
+++ b/src/common/dllinst.h
@@ -37,6 +37,7 @@ namespace Firebird {
 
 extern HINSTANCE hDllInst;
 extern bool bDllProcessExiting;
+extern DWORD dDllUnloadTID;
 
 } // namespace
 
diff --git a/src/common/os/win32/mod_loader.cpp b/src/common/os/win32/mod_loader.cpp
index 2d5ae12cf2..9c9e4fcbfd 100644
--- a/src/common/os/win32/mod_loader.cpp
+++ b/src/common/os/win32/mod_loader.cpp
@@ -226,7 +226,9 @@ ModuleLoader::Module* ModuleLoader::loadModule(const PathName& modPath)
 
 Win32Module::~Win32Module()
 {
-	if (module)
+	// If we in process of unloading of some DLL, don't unload modules manually
+	// else we could hang up waiting for OS loader lock.
+	if (module && !dDllUnloadTID)
 		FreeLibrary(module);
 }
 
diff --git a/src/jrd/CryptoManager.cpp b/src/jrd/CryptoManager.cpp
index 9febe3fc14..fd53c46bdb 100644
--- a/src/jrd/CryptoManager.cpp
+++ b/src/jrd/CryptoManager.cpp
@@ -332,7 +332,7 @@ namespace Jrd {
 
 			// ready to go
 			guard.leave();		// release in advance to avoid races with cryptThread()
-			Thread::start(cryptThreadStatic, (THREAD_ENTRY_PARAM) this, 0, &cryptThreadId);
+			Thread::start(cryptThreadStatic, (THREAD_ENTRY_PARAM) this, THREAD_medium, &cryptThreadId);
 		}
 		catch (const Firebird::Exception&)
 		{
diff --git a/src/jrd/os/win32/ibinitdll.cpp b/src/jrd/os/win32/ibinitdll.cpp
index 6b398757d8..7ea8106e93 100644
--- a/src/jrd/os/win32/ibinitdll.cpp
+++ b/src/jrd/os/win32/ibinitdll.cpp
@@ -47,6 +47,7 @@ BOOL WINAPI DllMain(HINSTANCE h, DWORD reason, LPVOID reserved)
 
 		case DLL_PROCESS_DETACH:
 			bDllProcessExiting = (reserved != NULL);
+			dDllUnloadTID = GetCurrentThreadId();
 			break;
 	}
 
diff --git a/src/jrd/svc.cpp b/src/jrd/svc.cpp
index b609027eed..7cb59c6362 100644
--- a/src/jrd/svc.cpp
+++ b/src/jrd/svc.cpp
@@ -849,7 +849,7 @@ void Service::detach()
 	if (localDoShutdown)
 	{
 		// run in separate thread to avoid blocking in remote
-		Thread::start(svcShutdownThread, 0, 0);
+		Thread::start(svcShutdownThread, 0, THREAD_medium);
 	}
 }
 
diff --git a/src/jrd/trace/TraceConfigStorage.cpp b/src/jrd/trace/TraceConfigStorage.cpp
index cc5252824c..499d3cd9c2 100644
--- a/src/jrd/trace/TraceConfigStorage.cpp
+++ b/src/jrd/trace/TraceConfigStorage.cpp
@@ -136,7 +136,16 @@ ConfigStorage::ConfigStorage()
 
 ConfigStorage::~ConfigStorage()
 {
+	fb_assert(!m_timer);
+}
+
+void ConfigStorage::shutdown()
+{
+	if (!m_timer)
+		return;
+
 	m_timer->stop();
+	m_timer = NULL;
 
 	::close(m_cfg_file);
 	m_cfg_file = -1;
@@ -153,6 +162,7 @@ ConfigStorage::~ConfigStorage()
 			m_sharedMemory->removeMapFile();
 		}
 	}
+	m_sharedMemory = NULL;
 }
 
 void ConfigStorage::mutexBug(int state, const TEXT* string)
diff --git a/src/jrd/trace/TraceConfigStorage.h b/src/jrd/trace/TraceConfigStorage.h
index 4a580dac0c..ca973c0567 100644
--- a/src/jrd/trace/TraceConfigStorage.h
+++ b/src/jrd/trace/TraceConfigStorage.h
@@ -65,6 +65,7 @@ public:
 	void acquire();
 	void release();
 
+	void shutdown();
 private:
 	void mutexBug(int osErrorCode, const char* text);
 	bool initialize(Firebird::SharedMemoryBase*, bool);
diff --git a/src/jrd/trace/TraceManager.cpp b/src/jrd/trace/TraceManager.cpp
index 9bfc154a2b..e1fc3aa638 100644
--- a/src/jrd/trace/TraceManager.cpp
+++ b/src/jrd/trace/TraceManager.cpp
@@ -163,6 +163,7 @@ void TraceManager::shutdown()
 			init_factories = false;
 		}
 	}
+	getStorage()->shutdown();
 }
 
 
diff --git a/src/yvalve/MasterImplementation.cpp b/src/yvalve/MasterImplementation.cpp
index 2257800ee1..830e48ddf3 100644
--- a/src/yvalve/MasterImplementation.cpp
+++ b/src/yvalve/MasterImplementation.cpp
@@ -43,6 +43,7 @@
 #include "../common/isc_proto.h"
 #include "../common/ThreadStart.h"
 #include "../common/utils_proto.h"
+#include "../common/dllinst.h"
 #include "../jrd/ibase.h"
 #include "../yvalve/utl_proto.h"
 
@@ -167,6 +168,7 @@ GlobalPtr<Mutex> timerAccess;
 GlobalPtr<Mutex> timerPause;
 
 GlobalPtr<Semaphore> timerWakeup;
+GlobalPtr<Semaphore> timerCleanup;
 // Should use atomic flag for thread stop to provide correct membar
 AtomicCounter stopTimerThread(0);
 
@@ -180,7 +182,7 @@ struct TimerEntry
 
 	static void init()
 	{
-		Thread::start(timeThread, 0, 0, &timerThreadHandle);
+		Thread::start(timeThread, 0, THREAD_high, &timerThreadHandle);
 	}
 
 	static void cleanup();
@@ -199,6 +201,7 @@ void TimerEntry::cleanup()
 		stopTimerThread.setValue(1);
 		timerWakeup->release();
 	}
+	timerCleanup->tryEnter(5);
 	Thread::waitForCompletion(timerThreadHandle);
 
 	while (timerQueue->hasData())
@@ -241,7 +244,25 @@ TimerEntry* getTimer(ITimer* timer)
 
 THREAD_ENTRY_DECLARE TimerEntry::timeThread(THREAD_ENTRY_PARAM)
 {
-	while (stopTimerThread.value() == 0)
+#ifdef WIN_NT
+	// The timer thread could unload plugins. Plugins almost always linked with
+	// dispatcher (fbclient.dll) thus, when plugin unloaded it decrement usage
+	// count of fbclient.dll. If application unload fbclient.dll not calling
+	// fb_shutdown, then last unloaded plugin will finally unload fbclient.dll
+	// and the code that is currently running, leading to AV.
+	// To prevent such scenario we increment usage count of fbclient.dll and 
+	// will decrement it in safe way at the end of the timer thread.
+
+	char buff[MAX_PATH];
+	GetModuleFileName(hDllInst, buff, sizeof(buff));
+	HMODULE hDll = LoadLibrary(buff);
+#endif
+
+	while (stopTimerThread.value() == 0 
+#ifdef WIN_NT
+			&& Firebird::dDllUnloadTID == 0
+#endif
+			)
 	{
 		ISC_UINT64 microSeconds = 0;
 
@@ -251,7 +272,7 @@ THREAD_ENTRY_DECLARE TimerEntry::timeThread(THREAD_ENTRY_PARAM)
 
 			const ISC_UINT64 cur = curTime();
 
-			while (timerQueue->getCount() > 0)
+			if (timerQueue->getCount() > 0)
 			{
 				TimerEntry e(timerQueue->operator[](0));
 
@@ -264,11 +285,11 @@ THREAD_ENTRY_DECLARE TimerEntry::timeThread(THREAD_ENTRY_PARAM)
 
 					e.timer->handler();
 					e.timer->release();
+					continue;
 				}
 				else
 				{
 					microSeconds = e.fireTime - cur;
-					break;
 				}
 			}
 		}
@@ -283,6 +304,19 @@ THREAD_ENTRY_DECLARE TimerEntry::timeThread(THREAD_ENTRY_PARAM)
 		}
 	}
 
+	timerCleanup->release();
+
+#ifdef WIN_NT
+	if (Firebird::dDllUnloadTID)
+		// fb_shutdown is called as result of FreeLibrary, not by application.
+		// Sooner of all we are the last user of fbclient.dll, and code will be
+		// physically unloaded as result of FreeLibrary() call.
+		FreeLibraryAndExitThread(hDll, 0);
+	else
+		// It is safe to decrement usage count here
+		FreeLibrary(hDll);
+#endif
+
 	return 0;
 }
 
diff --git a/src/yvalve/PluginManager.cpp b/src/yvalve/PluginManager.cpp
index 350ac7f04b..249ea8376a 100644
--- a/src/yvalve/PluginManager.cpp
+++ b/src/yvalve/PluginManager.cpp
@@ -41,6 +41,7 @@
 #include "../common/ScanDir.h"
 #include "../common/classes/GenericMap.h"
 #include "../common/db_alias.h"
+#include "../common/dllinst.h"
 
 #include "../yvalve/config/os/config_root.h"
 
@@ -1050,6 +1051,10 @@ void PluginManager::unregisterModule(IPluginModule* cleanup)
 	// and only if it's unloaded not by PluginManager, but by OS.
 	// That means that task is closing unexpectedly - sooner of all
 	// exit() is called by client of embedded server. Shutdown ourselves.
+#ifdef WIN_NT
+	if (!Firebird::dDllUnloadTID)
+		Firebird::dDllUnloadTID = GetCurrentThreadId();
+#endif
 	fb_shutdown(5000, fb_shutrsn_exit_called);
 }