Computer Reboots after startup

[mcdonger]

For up to an hour after it is initialy turned on, it is unstable, and reboots itself. (after it has been on a while it becomes stable) This appears more likely to happen if I am using greater system resources:
ie it happens less if I am reading emails, but almost certainly will happen if I am playing an online first person shooter game.

It is NOT a software problem, I am 100% sure, as I completely reformatted, and had the problem before running anything other than the OS (XP)

I currently have the case off and I am cleaning the very dusty fan, however as there is nothing monitoring the heat, I dont see why it would become stable after an hour or two, when surely it has become hotter.

Thanks for any help

[uesquebeathus]

almost all good manufactured motherboards have built in heat sensors, which are not quite 100% accurate but can help avoid burnout, but a dodgy power supply could also cause the fault if it is a low rated version and you are asking for more power than it can give, dirt and fluff on the fans and air intake areas like just behind the facia panel and in the fins of cooling blocks.