My Mac Pro has a tendency to freeze up now and then for a few seconds at a time, for no apparent reason. When it does this, iTunes seems to replay the last half second or so of whatever is playing, and it builds up getting louder and louder. Nothing on screen moves, no response form the mouse or keyboard. Then, after a few seconds, it just carries on like normal. I used to have an X1900 graphics card, which overheated and cause weird graphic anomalies, and I thought it was causing these hangs, but a while ago I replaced it with an 8800GT and I dust it regularly (just last week), but it still happens sometimes (today, for a particularly long time). Any idea what could cause this and how to prevent it?