Some time ago I had a problem that my PC was shutting down after being on for a few minutes, especially when I was in graphically rich applications. I noticed that my computer is not as noisy as usual, and I thought that I should check if the fans in my computer are working. I installed a temp. monitoring software and noticed that my computer was switching off when CPU temperature was 120C and this was the confirmation that I needed to know that this indeed was an overheating problem due to fan/fans not working. After inspection I noticed that the CPU fan was not spinning at all, and I replaced the whole cooler along with the heatsink and fan, while making sure that I buy the correct cooler for my computers socket type (in this case 939). I hope that this story may help you if you encounter similar problems.