Unfortunately, my random reboots discussed in this thread are back. I think it's probably my RAM, and I'm going to continue testing to try to find out by using one stick at a time and running memtest86 4 overnight.
(As an aside, I have tried setting timings to 1600/9-11-10-28/1.5V, I've tested heat on RAM and PSU using IR thermometer and measures cool, and I've tried running with case open. Everything is very cool, and I cannot find a heat issue anywhere. I think this is most likely faulty RAM. Could it be faulty CPU? Any other candidates?)
Meanwhile I am looking at getting new RAM. I'm just looking for stability at this point, I'm not trying to overclock my ram. I have a core-i5 2500K (Sandy Bridge) -- for reliability and stability, what voltage RAM is best? I'm looking at 1.5V and 1.35V sticks -- should either be just as stable/reliable, or is one voltage a safer bet than the other?
Also, there is a RAM compatibility list for my motherboard. I'm guessing it's a good idea to pick my RAM off that list -- yes?
I think I fixed the problem. I haven't been able to find any problems with my RAM. I noticed a correlation between vibration and reboot, so this afternoon I tested by hitting my case, and it immediately triggered a reboot. I concluded that something was loose.
I've opened the case up and put it on its side with the light on it and checked connections several times at this point. But I was determined to find what might be loose, so I tried again. Pushing and pulling on things I found my graphics card moved more than anything else. I unscrewed the card and found I was able to pull it out *without* moving the PCIe lock on the slot. So obviously it wasn't locked. I think reboots were from the card sliding around in there (even a tiny amount).
The card is big and heavy, and the CPU heatsink is huge, and there is very little space between them, so after I reseated the card and screwed it in again I couldn't get my finger in there to pull the PCIe lock. I tried with a screw driver but it wasn't long enough and I was worried I'd damage something if I was pushing hard and it slipped. So I cut a piece of string and made a loop and tied a bowline knot, and went fishing. After I got it hooked on the lock, I eventually go the end of it slid through a hole in the back of the case so I could pull it the right direction. I pulled, the string snapped, but the lock was in place.
No reboots so far! I closed up the case and resumed my overclock (lower overclock on the RAM since I'm now running at the proper 1.50V instead of 1.65V -- 2133 MHz wasn't stable). I'm running Prime95 now.
Moral of the story: if you have random reboots, triple and quadruple check your connections, including graphics card.