Flash video won't stay full screen in Windows 7 retail

[Rehevkor]

Since I did a clean install of Windows 7 Home Premium, flash videos embedded in web pages will not play full screen properly. I can get them into full screen for about 10 seconds, then they just pop back out of it. This happens on various websites (youtube, gametrailers, Netflix instant watch) in both Firefox and IE. Flash video played in full screen just fine when I was running the Windows 7 RC. I've tried reinstalling the latest version of flash, but that had no effect.

Any ideas?