Computer Hung, and then when restarted, the cpu clock decreased.

Hi all,

need help please with my computer problem, here's the case;

My computer spec are:
- CPU Intel Core2Duo E6750 (2,66 GHz )
- Motherboard Asus P5K
- Memory 4x Team 1GB pc 6400
- GPU Nvidia 9800 GT ( Pixelview brand )
- HDD WDC 1TB Sata
- Power Supply FSP 400W
- Asus DVD-RW Sata

this computer is about 6 years old, running Windows XP SP-3, and it's

not overclocked.

In random time, while in idle time or gaming, the computer hang, no

mouse cursor movement, no BSOD.
Therefore, usually i restarted by restart button on the computer.
When restarted and enter the boot screen, the cpu clock stated lower

than it should ( 2,66 GHz ), about 1,4 GHz. and the BIOS said that the

setting in the BIOS was wrong.
So, i enter the BIOS and do "Load setup defaults", then i change again

to my previous settings, which was the boot logo and the boot sequence

only.
Then it run normally again. The hang of the computer happen 1x-3x at a

time, with the "Load setup defaults" done only once.

What is the problem with this computer? please help. thanks.