What is the most likely cause
Motherboard North bridge overheating.
who do i best fix it?
Try running it with the case completely off with a room fan or some other ventilation source moving air at it. If that seems to solve the problem, cleaning the case fans/intakes with canned compressed air or the like probably fixes it. If that doesn't solve the problem it could be a myriad of other things. It's usually heat, though.
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