Sounds like it's overheating. Especially if it shut down quicker the second time. Likely it hadn't fully cooled off and shut off quicker. If this is the case, maybe you should look at how you attached the heatsink. If you used the stock heatsink, check to see if there was a protective plastic sheet over the TIM. If there was, the protective sheet has to be removed so the TIM comes in contact with the heatspreader on the CPU.