Most frequently in Firefox 4*, but also with other (non-web) applications, the system will randomly hang in the middle of an action. Timing varies from something as simple as typing, to clicking a link, or switching applications. There doesn't seem to be anything consistent about it. It might happen every couple of minutes or it might go an hour between showing up. When it happens, it will spin for 1-3 minutes, then disappear. Or sometimes the computer will freeze up and I have to try to do something (even just clicking the trackpad button) to prompt the beach ball. Sometimes, after the beach ball goes away, the computer will still be hanging, though. So I have to just let it sit for a few minutes until things return to normal. I've tried using Activity Monitor, but usually the system freeze prevents me from seeing if anything is not responding (ie. I can't switch to it until the freeze ends). Same problem with attempting a forced quit. Also, if it hangs long enough, the fan will start running heavily. I'm running OS 10.5.8 on the MacBook in my signature. *In Firefox, this is sometimes (but not always) accompanied by an unresponsive script message.