What do you mean exactly? Do you mean that it reverts back to display only on the computer, even for a few seconds? Are you using Powerpoint in dual-screen presenter's mode (full screen on the projector, image, comments and next slides on the main screen)? Are you displaying movies or playing sounds when problems happens?
1. Until you talked of various projectors, I thought it was a problem on that end. But are you using the same cable (or cable extension / adapter) to link your computer to the projector? If so, I suspect the problem would lie in the cable. In our office, we had many projection problems with travelling personnel (on-and-off projector, weird colours, etc.), and in all cases but one it was a faulty cable. If you are always using different cables (and no dongle / adapter of your own), then it might be your output port. If you have the choice between VGA port and HDMI or display port, try to connect a projector using the other port to see if it works reliably.
2. If you have intermittent problem, Powerpoint's presenter's mode doesn't help because you not only have to deal with hardware problem but with the quirky reaction of Powerpoint. Using the normal mode with both displays showing the same thing would help a bit go around the problem.
3. I have noticed that Powerpoint sometimes has problems dealing with too large or too complex videos. What "too complex" means depends on a combination of factors like how much RAM, your computer speed... I would think it is the issue especially if point 4 is a problem.
4. It may be hard to diagnose if the problem is very intermittent. But does it happen only with Powerpoint or also with Word, Excel, Acrobat, etc.?
