Sort Top by CPU usage by pressing capital "P". What percentage is the greediest process sucking down? The processes will jump back and forth, because the one using the most will change all the time. You should however be able to identify one that is greedier than the rest, if you have a "runaway" process.

You can sort by memory usage by pressing M.

Do you have flash enabled websites open in "background" windows when this occurs?

If you want screenshots, I have a short "tutorial" at my blog (sorry for spamming guys). n900experience.blogspot.com.