Please help my pc crashes after start

[sabertooth326]

hi guys i have crashing problam with my pc after boot windows shutdown about 10 sec after showing my desktop i dont understand i didnt do anychanges and i ran test every hardware past it i did reinstall os but it didnt help, i relised that my cpu clock changes randomly after each boot !! my system is
phenom x3
4gb ram
ati 5850
680 watt psu

 

[saint19]

Hi.

1- What OS do u have?
2- Do u get some error (BSoD) or something more?