Couple of things to check. Make sure all your fans are working, and install a program like Motherboard Monitor to check the temps. You can also use it to check the voltages from the power supply (5V and 12V).
Unplug any PCI cards and USB peripherals (sounds card, modem, etc), especially any network cards.
Reset your BIOS setting to default.
If none of the above have any effect, you likely have a bad motherboard. Inspect it, paying close attention to the capacitors. Look for any bulging or leaking. This has become an all too common of problem.