I'm running Windows 7 Home, so don't know if this relevant at all. I have had similar problems as you describe with the GTN750/650 combination in the past, and discovered that they seemed to occur only after having used the computer for something other than flight simming, surfing the web, for example. On cold starts and directly loading up P3D, there were no problems. So from then on, if I want to sim, I do so without running any other program which is not related to flight simming. Loading up weather, Little Nav Map, etc., has no effect on the functionality of the GTNs, of course.