-1 on flash. I've had similar problems with flash sites and streaming online. It works much better these days (no clue why) on my 2007 macbookpro. Didn't bother with Lion though.

 

Its flash and yes us PC users have problems with flash too.

 

You can force youtube to use h264 or HTML5 via certain browser extensions.

Some of my coworkers really like ClickToFlash, which essentially blocks flash unless you click to enable it. They say that on news sites it helps as much of the content beyond just the video is flash based, and when all of it is running that is when they have issues.

You could also try another browser, such as chrome or firefox (if you use safari currently that is).

Although like Simba said, flash sucks on the mac.

 

To the OP, it's indeed Flash. Try to uninstall it and reinstall it.. I just had to do this on a friend iMac - was a real pita. What makes it worse, sometimes you get a bad download of Flash which still causes the problem.. I tried a couple times before it finally went through with no problems..

 

Hoss, have you downloaded the latest OS Update? One of the issues addressed in the update is the system being affected by video.

Try that out and see if it helps.