Occasional BitBlt failure, GetLastError=0

I'm seeing a very similar problem with my BitBlt's. It will return FALSE only occasionally (once every few hours, being called a few times a second) and an immediate call to GetLastError returns 0.

MFC in VS .NET 2003. I don't know what Hyperthreading is or whether I have it enabled, though.

We have a dialog app that connects over TCP/IP to a server and receives video data. On the data receive callback, we BitBlt the image into a CDC that was created in initialization using the following:

            CDC* clientDC = this->GetWindowDC();
            if(clientDC == NULL)
                  throw -1;

            if(! m_imageMemDC.CreateCompatibleDC(clientDC) )
                  throw -1;

            if(! m_imageMemBitmap.CreateCompatibleBitmap(clientDC, CAM_W*m_numCamW, CAM_H*m_numCamH))
                  throw -1;

            if( (m_pImageMemOldBitmap = m_imageMemDC.SelectObject(&m_imageMemBitmap)) == NULL )
                  throw -1;

            this->ReleaseDC(clientDC);

A Timer control is used to send a WM_PAINT every second. OnPaint, we StretchBlt the m_imageMemDC to the screen, as follows:

      CDC* clientDC = this->GetWindowDC();
      if(clientDC == NULL)
            return;

      clientDC->SetStretchBltMode(COLORONCOLOR);

      EnterCriticalSection(&m_imageMemDCSection);
      clientDC->StretchBlt(0, 0, clientWidth, clientHeight, &m_imageMemDC, 0, 0, CAM_W*m_numCamW, CAM_H*m_numCamH, SRCCOPY)
      LeaveCriticalSection(&m_imageMemDCSection);

      this->ReleaseDC(clientDC);

As you can see, we use a CCriticalSection, and lock it every time we access m_imageMemDC. Here's the BitBlt code:

            EnterCriticalSection(&m_imageMemDCSection);
            if(! ::BitBlt(m_imageMemDC.GetSafeHdc(), posX, posY + GetInfoBarHeight(), width-2, height - GetInfoBarHeight() *2,imgPtr->GetDC(), 0, 0, SRCCOPY))
                {
                  CString info;
                  info.FormatMessage(IDS_E_BITBLT, _camID,  GetLastError());
                  theLog.Log(info, EVENTLOG_ERROR_TYPE, 0, 0, CRITICAL);
                  TRACE (info);
            }
            LeaveCriticalSection(&m_imageMemDCSection);

As previously mentioned, we get the eventlog error, which includes the GetLastError() return code as '0', on average once an hour, but not at a regular interval.

Why does BitBlt fail?

Thanks.