Ugh, mine does that sometimes and it's annoying. In my experience, it does it because I have too much running in the background. For example, when I work with Fireworks, I'll have my headphones on, with WinAmp running. And maybe I'll have Yahoo Messenger up, talking to friends in between projects, etc. This will sometimes lag me. But not often.
Check your computer's resources. If you hit Control + Alt + Delete (don't hold it down), it should bring up a message showing everything that you are running in the background. If you have a whole list of things, you need to check to see what's on your Start Up menu.