I built a computer for my friend.
When I was building the computer, I did notice that the computer would randomly reboot during OS install. So we replaced the PSU from the stock 300W case PSU to the Rosewill.
Now he's complaining that it randomly reboots.
I told him to loosen some of the MB screws, still reboots.
He replaced the CPU fan with CoolerMaster Hyper 212+. Reboots, but it happens less.
So I brought the computer home and ran orthos to stress it and make it reboot.
Ran the SmallFFTS-Stress CPU for an hour, no error. Ran the Large in-place FFTs-stress some RAM for 20 minutes, no error. Ran "Blend-stress CPU and RAM" then it reboot after a couple minutes. After reboot, ran it again and it gave me an error saying the test failed after 5 minutes (but no reboot). Couldn't find stress.txt so can't tell you what it said. Ran the test again in blend and shows me the same message after a couple minutes.
So I thought, maybe it's the RAM.
Ran MEMTEST86+ for about half an hour, and it reboots again.
Not to sound condescending, but if you don't have a clue as to what's going on or how to find out, you perhaps should not build machines for others just yet.
Random reboots can be hard to track down, but it sounds like you've started on the right track. It's obviously not CPU heat as Prime95 or Orthos would have exposed that. Memtest86+ is a good test. If the machine is power cycling during that test, the issue is almost certainly RAM. The best way to test this is to use it in another machine, and test again w/memtest in that box. This will expose the issue. If RAM does work flawlessly in another box, then your issue could be one of three things - An electrical short (not likely, but possible), a faulty motherboard (hard to diagnose w/o replacing the thing in the first place), or a shitty PSU (Rosewill makes junk, this is not unlikely). For most commercial grade electrical components a 5%+/- tolerance is allowed on voltage. Sometimes not, but usually there is a %5 threshold. It is entirely possible that the power supply in this computer is fluctuating more than 5% on the 3.3, 5, or 12v rails during operation. The best way to find and resolve that issue? Replace the PSU with one that is known to be good and can provide enough power for the current system.
To sum it up - RAM probably is the issue. Barring bad RAM, replace the PSU with a known good unit if you have one available. If that fails, the motherboard is probably at fault.
One other thing... I did notice that when it's been unplugged for a long time and I plug it in and press the power button, it won't start right away. It needs to take a few seconds to "charge" or something before i press the power button.
PSU related or MB?
When I get home after work today I'll try swapping RAM and then the PSU if necessary.