Python/C++ timer intermittent bug

[Paul]

I have a problem with threading using the Python/C API. I have an
extension that implements a timer, and the C++ timer callback function
calls a Python function. The relevant code looks like this:

static PyObject *timer_setmodname( PyObject *pSelf, PyObject *pArgs )
{

	char *b;
	PyArg_ParseTuple( pArgs, "s", &b );
	mod = PyImport_ImportModule(b);
	if( mod == NULL )
	{
		printf("Could not import %s\n",b);
		return Py_None;
	}
	modsetFlag = TRUE;
	return Py_None;
}

static PyObject *timer_setprocname( PyObject *pSelf, PyObject *pArgs )
{
	char *b;
	if( !modsetFlag )return Py_None;
	PyArg_ParseTuple( pArgs, "s", &b );
	attr = PyObject_GetAttrString(mod,b);
	if( attr == NULL )
	{
		printf("Could not import %s\n",b);
		return Py_None;
	}
	attrsetFlag = TRUE;
	return Py_None;
}
	
static void CALLBACK PeriodicTimer(UINT wTimerID, UINT msg, 
    DWORD dwUser, DWORD dw1, DWORD dw2) 
{ 
	PyGILState_STATE pgs;

	pgs = PyGILState_Ensure();
	if(attrsetFlag)
	{
		pres = PyObject_CallFunction(attr,NULL);
		if( pres == NULL )printf("CallFunction failed!\n");
	}
	PyGILState_Release( pgs );

} 

The Python code that sets this up looks like this:

fetimer.setmodname("Timeslice3")
fetimer.setprocname("Timetester")
print "\n Program Waiting for Time Slice"
while True:
    time.sleep(0.010)


and the module Timeslice3.py looks like this:

#Timeslice3.py
def Timetester():
	pass 

When I run this stuff, it works fine for hundreds, often even
thousands, of timer ticks (I've been testing with about thirty ticks
per second, but it doesn't matter - it still crashes at ten or fewer
ticks per second). Sometimes it runs for only a few seconds, sometimes
for ten minutes or so. But it always eventually crashes Python.
Usually it gives no error message. Sometimes, though, it does give an
error message, but not always the same one. I've noted three that it
has given in my testing so far:

Fatal Python Error: This thread state must be current when releasing

Fatal Python Error: PyThreadState_DeleteCurrent: no current tstate

Fatal Python Error: PyEval_SaveThread: NULL tstate

Can anybody help me make this code stable, so that it works all the
time? I'm using Python 2.6.5 under Windows Vista, but it crashes under
Windows XP as well.