[Xrati]

My guess is that the processor is over heating (it doesn't have to be by much to cause a shutdown). As the time is different between shutdowns and the heat is a relative function of how hard the load on the processor. If your N is ½ hr. to six, is the ½ hr. after it was already running and shutdown? Is the 6 hr. N when it was running from a cold boot?

Take the cooling fan off and clean the surface of the processor and bottom contact surface of the fan Make sure you clean the fan too!