Where do I go, follow the river?

We've been having some problems with YouTube videos disappearing. At least, on the two Macs used most often in the Audio household. After some actual, you know, troubleshooting, it seems to be a Mac thing in both Safari & Firefox. The video will appear after the screen sits for a while, but scroll or page up or down, it disappears.

If anyone else notices similar weird goings-on, please let me know in comments.



Thanks!