Sounds like an overheating CPU.
Might be worth remounting the CPU and cooler just in case. Possibly the cooling grease has shifted during the the first few days of use.
Try touching the sides of the cooler just after it switches off to see how hot it is to touch. The thermometers in the mobos are not always accurate.
Have you got a lot of other internal devices, multiple hard drives, high spec gfx etc. You could be stressing the PSU if it is only 300W or so.
Try running it with something disconnected from the power, a slave CD or something else you can live without for a day.