Can't change voltage above 1.25

I've just been messing around overclocking my system but I can't seem to change the cpu voltage above 1.25v.

System Specs (stock): CPU - x2 4400 (brisbane) core speed - 2311.0 mhz multiplier - x11.5
HT Multiplier - x5 HT Frequency - 1005 mhz (stock fan btw)

Motherboard - M2N e Sli PCI E clock - 2500 mhz

Memory - 2 x 1G Kingston DDR 2 (800)

GPU - Asus 8600gt 256mb core - 540 mhz memory - 700 mhz


I've overclocked it before to about 2.6 mhz (cpu) HT Frequency (1200 mhz) Memory bus DDR 2 (960)
PCI e clock (2925 mhz) I haven't changed any of the memory timings.

It seems stable but when I put on a good load it will crash after about 15 mins.

I'm not allowed to change the voltages above 1.25 in bios or n tune (I can't change the voltages in n tune at all)


Is there a solution to this problem