weird.. your not on insane volts.. at 4000 with 8 multi CPU-Z shows 1.296v.. thats not very high.

and my machine at 4.3 Ghz on 1.3456v fails Small FFT's instantly, but blend goes for about 10 mins then system crashes BSOD. and Large runs for aboug 5 mins. but I'm still tweaking and need better coooling.

but I've managed a 478 FSB on my 750i and I'm sure I could get it stable with the right tweaks.