Ok, I'll definitely be looking into getting my hands on at least a temporary replacement PSU to test against the current one. I just saw your newest post moments after writing the results of my tests w/ the 81.85 driver, so I'll leave those results here to for perusal.
In regards to the question of if the PC locks up/shutsdown when using the 81.85 nVidia drivers, not yet. I wouldn't put it past it because I /have/ had whole days even a few days w/o a crash, but then out of nowhere bam. However, with the 81.85 driver in place, there are two notable issues that are constant. The first is when playing WoW in SLI mode, if I alt-tab out of the game, and go back in, I get a flickering tint at the top 1/3 of the screen, that reaches down to 2/3 of the screen on some flickers. When I enable 'Show GPU load balancing' in the SLI multi-GPU options, I find that I /can't/ alt-tab out of WoW at all. Upon pressing alt-tab, the screen freezes, and I get the windows cursor, but I'm stuck moving it around on a stillshot of my game. Alt-tabing again to get back into the game creates the same screen flicker tint mentioned before. Running WoW while in single GPU mode causes no problems.
Testing Battlefield 2 reveals similar issues. I can not properly alt-tab out of the game. I am allowed only to move the normal windows arrowhead around on a still image of my game. However, alt-tabing back /into/ BF2 from this state doesn't mess up the graphics with a tint-shearing flickering effect like in WoW, however on one occasion the whole screen would flash all black for spells of around a second, a few seconds in between. Restarting the game fixed that. And again, running this game in single GPU mode results in no problems from the testing I've done.
Voltages still fluctuated from what I could decern between shutting the games down and starting them up.