What is causing fluctuating cpu usage spikes when using firefox?

Bumping this thread because I came across it via Google. I had the exact same problem (Firefox CPU usage spikes every 10-15 secs, Flash videos freezing for 1-2 secs) and solved it by deactivating and reactivating the Flash plugin.

Before doing that my CPU load looked like this:



I went ahead and deactivated the Shockwave Flash plugin in the Add-On options in FF, killed the Firefox process using the task manager, restored the session, reactivated Flash, killed the process and restored the session again and now it looks like this:



Flash video plays without a hitch now.

 

I just had to join to say that this was a miracle cure for me! It got rid of the CPU spikes like nothing else! Thanks so much!