[sswilson]

If I had to guess, it'd be NB voltage(s), but it could be a number of things.

I had a bugger of a time with random shutdown(s) with my 790FX and 890FX boards on OCs which had already done 10+ hours of Prime95 stability. The key (I think) is the "have to power off the PSU to get a post" symptom.

I did 3 things to resolve the issue (one of which obviously worked as I haven't had a repeat in well over a month of 24/7 folding)...........

1/ On the off chance that bios was "tightening" the secondary memory settings too far, I manually set all of the secondary timings to a known good setting.

2/ Since I was OC'ing the NB, I had concerns about the ratio between HT and HTT (NB). I had initially tried to keep HT between the 1800 - 2200 listed as spec, but since most freezing (in my experience) is related to either low NB voltage, too high NB clock, or Memory I decided to bump the HT higher than rated.... currently running a NB around 2800 with an HT of around 2400. On the NB/Memory front I also bumped my NB voltage(s) up a notch and set my Mem Voltages up a notch to compensate for using 4 sticks vice 2.

3/ Lastly.... as with all "stable" OC's, there's always a chance that no matter how much stress testing you do, it might still be unstable..... as such I bit the bullet and decided to dial my OC back from 4180 to 4120.

After having fought with some of the same symptoms you are describing (my issues were usually random shutdowns rather than freezing, but getting it to post required turning off the PSU as well), I'm happy to report that one of these steps did the trick and I'm now fully 24/7 stable.