Applied new thermal paste, PC shuts off after a minute

[Bearson]

I have had my computer for almost 3 years now and today I decided to clean out my tower of dust when I noticed recently that my computer was randomly shutting off while playing games(Starcraft 2/Portal 2). I opened the case, used comprssed air to clean it out. Removed the heat sink, cleaned it, and put it back (Note: I have done this whole process twice before and everything worked). Now when I turn on the computer it stays on for about a minute and 20 seconds before shutting itself off. The only thing I did different this time was remove the old thermal paste and apply some new paste (Arctic 5 Silver).

Plus now the monitor won't recognize the computer so I can't check the temperatures before it shuts off.

Yes I checked to make sure the heat sink is all the way in. It can't get any more tighter or secure.

All computer fans turn on as they normally would.

I removed the video card (ATI Radeon 4850) and it still shut down so the problem doesn't seem to be the video card.

Just in case I put too much thermal paste on, I removed it once again and put less on the second time.

Motherboard is an ASUS P5N-E SLI

 

[Noith]

I'm having the same issue. I got an old tower from my brother that needed a new power supply, and it worked for about six hours straight with no problem. After moving it to a new outlet, it started shutting off without warning. Happens faster every time I boot it up, but the temp stays around 83 degrees according to the on board thermometer. Not sure what to do with it.