I've also just started having this problem with a 2 year old Core Duo 2 MacBook Pro. Mine just shutdown after the machine was on for less than 5 minutes, so I doubt it has anything to do with overheating, at least in my case. I already reformatted the drive and restored my data just a week ago.

I did notice on startup this last time that the monitor image was fairly whacked for a second before coming into view properly. I would guess a logic board issue, as well.