I also have had the system both freeze and random reboots. Usually it has been accompanied by severe video corruption, including flickering, and then a freeze where the mouse moves, the system still seems to be running but the screen does not update. Only a hard power button reset seems to brings it back.
Here is a good general mac maintenance/troubleshooting article for all of you.
I'd suggest you all read it and go throught the steps that are suggested.