Problem with unlocked 550BE

Hello.

I just got a phenom II x2 550 which i unlocked to x4 B50.
The motherboard used is Asrock K10N78D.

The CPU is stable in all applications ( cinebentch, performancetest, etc ).
But when I stress it with wPrime the OS locks after 2sec and I have to reset it.

I thought a core was damaged but running wPrime on any combination of 3 cores works, so as long as any 1 core is idle the test works.
5v and 12v were measured and they remain stable.
Extra CPU voltage doesn't seem to help.
The cpu MOSFETS get very hot during testing.
The motherboard is rated to handle processors with TDP of 125W.
Phenom II x2 550 is 80w.


What could be the problem / what do you advise?
Thanks.

I'm not familiar with that board - can you set the cpuNB voltage and multiplier in BIOS?

I'd guess the cpuNB is weak if it'll run ANY three cores the same but not all four. Adjusting the cpuNB voltage and/or speed might resolve the issue.



Edit:
If the MOSFETs are getting hot I'd suggest heatsinks for them. Personally I don't like OC'ing a board without MOSFET coolers or, at a minimum, a dedicated fan ...

After testing some more it turns out that the problem is caused only when testing Prime Large FFTs( stress RAM ).

I decreased the cpu clock from 200 to 180 resulting in an actual CPU clock of 2.8 ( from 3.1 ). I raised the NB voltage from 1.15 to 1.20 and raised the CPU from 1.35 to 1.45, but after 10 or so seconds of Prime95 large FFTS the system freezes.
I installed heatsinks on the mosfets and now they run verry cool < 40C.

I can do Prime95 small FFTs with 1.45v @3.6 ghz stable.
Why is prime Large FFTs unstable even when underclocked & voltages raised?

But you're still running more than the stock two cores, right? I mean even though it's under-clocked the CPU still isn't stock - you've unlocked cores that the factory locked off because they wouldn't pass the factory tests.


Or are you saying all this is happening with the stock configuration???



Edit:
Half the memory sub-system is on the CPU, more specifically, it's on the cpuNB. Also, I doubt small FFT's stress the L3 cache much and it's also on the cpuNB ...