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This one has been torturing me for a while.

Managed to track it down to a problem inside Tcl.
For the Tcl8.3.4 source distribution the problem is in
the file win/tclWinNotify.c 

void Tcl_FinalizeNotifier(ClientData clientData)
{
    ThreadSpecificData *tsdPtr = 
                         (ThreadSpecificData *) clientData;

    /* sometimes called with tsdPtr == NULL */
    if ( tsdPtr != NULL )
    {
        DeleteCriticalSection(&tsdPtr->crit);
        CloseHandle(tsdPtr->event);

        /*
         * Clean up the timer and messaging 
         * window for this thread.
         */

        if (tsdPtr->hwnd) {
	    KillTimer(tsdPtr->hwnd, INTERVAL_TIMER);
            DestroyWindow(tsdPtr->hwnd);
        }
    }

    /*
     * If this is the last thread to use the notifier, 
     * unregister the notifier window class.
     */

    Tcl_MutexLock(&notifierMutex);
    if ( notifierCount && !--notifierCount ) {
	UnregisterClassA( "TclNotifier", 
                          TclWinGetTclInstance());
    }
    Tcl_MutexUnlock(&notifierMutex);
}

This bodge doesn't address the underlying problem but has 
stopped me from tearing all my hair out,

cheers,
John Popplewell.