I've taken the Vcore on my i7 920 D0 down from 1.217V (auto) to 1.005V and so far it has been running stable on a Prime95 In-place Large FFT Torture Test @ 133.5 x 21 with HyperThreading on. The biggest difference I've noticed is a 12ºC reduction in peak CPU temp.
I'll wait for the test to run for a day or so before I declare victory.
Though its as hot as Hell itself here atm so it is still running quite hot, though it would be running @ well into the mid 70's ºC on the stock voltages, instead of the low-mid 60's ºC it is currently running @.
I think 48 hours is long enough to declare victory. I full two days and my PC remained stable, even while I was surfing the web, ripping DVD's and watching movies at the same time.
The problem now is that because I've set a custom voltage, the voltage no longer varies with CPU load, so the idle temp is now slightly higher than before and I can't find a way to get the voltage to vary between a custom min and max.