I don't mean to start a new Thread but I think this one could really help pinpoint the problem if we could get some good feedback on this. What is the Common Denominator (besides having the HD4850 option) that could be causing the lockups that some people are having. Examples Is everyone who is experiencing lockups : 1.) Installed Boot Camp and using Windows with this HD4850 option? 2.) Do you use Safari exclusively and not FireFox? 3.) Does it happen only when Surfing ? 4.) Does it happen only when playing a certain game like WOW? 5.) Does it happen only after you been using the iMac for a few hours? so on. . . See my point? Write down or make yourself notes to as what you were doing exactly when it locked up? Reply to this Thread and lets figure this out. It will be must easier if everyone could put their situation in this Thread and maybe we can get to the bottom of this. Look forward to your feedback.