This may be a general hardware failure unrelated to the CPU, but assuming your processor is the problem, underclocking it will not help in the long run. It sounds like your power supply may not be providing enough power to the CPU, therefore crashing your system periodically during periods of high CPU load. Since your CPU fan is working, I doubt it would be an overheating problem. Be sure to check your voltage settings in the BIOS to make sure they haven't been modified incorrectly.
If you can provide any other information about the hardware in your system, it would be greatly appreciated.