IUnknown pointer error, exception

Hi,

I need to read an XML file which holds input data for a Service into a DOM tree. The main process initilizes everything and sets the start data and a update thread need to check for changes in the XML file and should update the DOM tree.
I setup an event to prevent conflicts during access of the DOM tree. The program runs fine, I get all my data out of the DOM as expected, but my problem is now, that after the 3rd run of the Update Thread, the IUnknown pointer of the m_docPtr is invalid and when triing to use it I will get an exception. I reduced the code and just put an QueryInterface in the routine which should normally update the DOM tree and still I have the same problem.
So I don't use the pointer at all no more just for QueryInterface in the update routine and I still have a invalid IUnknown pointer inside of the m_docPtr.

The m_docPtr is a member in a class which is passed to the thread. I tried both pointers MSXML2 and COM, async and sync access, msxml3 and msxml6 docs, since msxml3 throws according to microsoft an exception during heavy traffic.
But I still get the same result after the update thread runs the 3rd time the pointer is invalid and throws an exception when I use the m_docPtr

class CmyDOM

//MSXML2::IXMLDOMDocumentPtr m_docPtr;//pointer to DOMDocument object
CComPtr<MSXML2::IXMLDOMDocument> m_docPtr;


Main calls Init in the Process in main Thread which runs till the Service is terminated

Init:

m_DOMAccess.Lock();

try
{

m_docPtr.CoCreateInstance(__uuidof(DOMDocument60));
m_docPtr->put_async(VARIANT_FALSE);
varResult = m_docPtr->load(m_varSrcFile);
if (!(BOOL)varResult)
{
Trace(ERROR,"CmyDOM: failed to load XML file. Check the file name." );
m_DOMAccess.Unlock();
return 2;
}

}
catch(...)
{
m_DOMAccess.Unlock();
return -1;
}



Update Thread:


DWORD WINAPI UpdateDDTThread (LPVOID lpParameter)
{
CmyDOM *pmyDOM = (CmyDOM *)lpParameter;

while (CmyDOM->UpdateThreadCmd != TS_STOPPED)
{
CmyDOM->UpdateDOM();
.....

Sleep(Delay);

}
//thread terminated
ExitThread (ERROR_SUCCESS);
return ERROR_SUCCESS;
}

Does anyone can help me with this. What is the proper way to update a DOM tree when the input XML file is changed?

Thank you in advance.

Martin