I have been having problems with my guide camera and recently opened it up and did some testing and am seeing something related to what BillF is describing. I made a video that shows the behavior. Maxim is set to take continuous guide camera frames. I can see that the main camera shutter is opening as well as the guide camera shutter even though I have not asked for this. I can also see that the main camera shutter does not seem to stay in sync - once in a while the guide camera shutter does a 'double take'. This seems to happen every 7-8 frames. It happens when the guide camera shutter and the main camera shutter have fallen out of sync. I don't know the cause, just the symptom I made a thread about this in the Maxim area along with another issue I'm seeing that caused me to open up the filter wheel. I would post the video but the forum software is refusing to let me upload it.