check all the heat transfer pads (esp the south bridge one) are still in place
if the south bridge one is missing then usually the fan blows fast right from powering up then shuts off.
it could be that its just shutting off for you and missing out the fan.
but figured i'd mention it as your re-opening to check the dutch er i mean fuse
the other thing will be cracked solder under the nor legs or a compoment missing next to the nor you rough bugger lol