A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
It's possible that this is a hardware problem that's triggered by something PPT's doing.
For example, depending on your slide show settings, PPT might be changing the video resolution.
Check the Slide Show tab on the ribbon and look for monitor resolution settings; if it's not set to Use Current Resolution (or words to that effect) change it to that, or at least set it to whatever resolution your monitor's currently set for. Then try the slide show again to see if this helps.