CPU_NB sounds like its a problem, MAX for C2 chips being around 2600 with ~1.4v (default multiplier at 260 bus gets you this number exactly).
But C2 chips almost always, from what I have seen, crap out at around 3.7-3.8. I have two C2 Phenom II's and both hit a wall around that clockspeed. Even many more volts dont help much.
And its not a thermal limitation either, both chips are running at less than 50c on the socket and less than 40c on the core sensor.
HP-UX. I came, I saw, I was sad it would not run on my X86 box. [Official]AMD RX 480/ 470/ 460 Owners Club