corrupted bios

[Rolo42]

Sounds like you're hitting max Tcase (CPU case temp) and its down-clocking to prevent overheating.  This will happen with the stock cooler; what cooler are you using?

[kostas75#]

corsair h 80 , the higer cpu temp while prime 95 is running is 71 degrees,i think that something else is overheating,not the cpu.

[Rolo42]

Southbridge doesn't get hot enough and wouldn't affect the CPU clock anyway.

Memory overheating would cause BSODs.

What is the temp of the coolant?  If its not proportional to Tcase, then I'd check the heatsink-to-CPU connection, radiator fan, pump, or fluid flow/level.

[kostas75#]

I discovered something,the thing that makes motherboard to go back to 3,2 ghz is the overheating of the power supply of cpu, i placed a fan hitting that circuit and for 1 hour with 100% load cpu frequency is stable!!!!