Flash not working on Macbook

I have a Macbook, 2Ghz Intel Core 2, 2 GB Ram, running OS 10.7.3. Safari 5.1.3, Firefox 12.0, Flash 11.2.

Flash based videos etc. aren't working. I'll get a spinning beach ball, then "plug in failure". This happens on everything from the Yahoo homepage, to YouTube videos.

I've tried logging in as a different Admin. user. No change.

I've reinstalled Flash. No change.

I've reinstalled the OS. No change.

Next?