Flash on OSX Lion is a TAD BIT BUGGY right now. That's not lost on Adobe, and they've posted a document of all known issues with 10.7. One issue they suspect, but can't prove: Apple disabled hardware acceleration with Flash.

Flash Player may cause higher CPU activity when playing a YouTube video. Possibly related to disabled hardware acceleration.

Other issues include the inability for the mouse to interact with dialog menus in Flash, and custom mouse cursors not behaving as they should. [Adobe via TUAW via 9to5mac]