If the fan is spinning quickly to look at - i wouldn't worry about exactly how fast. However, especially if you have reseated the cpu, check the thermal paste hasn't gone funny - respread it around the cpu and heatsink in a thin layer with the edge of a credit card or similar - this is a quick and dirty fix not something that is best practice though (should clean and renew but if just fault finding....). Also give it all a good clean and check the capacitors on the motherboard - if they have leaked from the top then the motherboard is shot. Also reseat the ram, and try one at a time in case of a faulty module. when cleaning pay special attention to the cpu heatsink, if the fins are clogged it will overheat - but cpu's are quite resilient really - the shutdown tends be a safeguard for it.