My present problem has been really tricky to pin down, so here's my system and I'll try my best to describe the problem. Hopefully someone can shed some experience on the situation.
Motherboard - Asus P8P67 Deluxe Rev. B3 Bios 3207(04/02/2012)
PSU - Antec neo eco 620 watt
CPU - intel i5 2500k (oc @ 4200)
RAM - G-skill DDR3 1600 (2x4gb + 2x4gb - I bought one lot first then upgraded later so the 4 sticks aren't of the same batch)
HDD - WD 1tb Caviar green
Video - gigabyte 1gb AMD 6850
Cooler - Coolermaster Hyper 212+
Additional cards - T-link wifi pci card and 1 case fan
OS - Win7 x 64
Mid last year I build a new computer (about a year ago) it ran fine, like a dream for about 6 months. Then one day I left my computer on to get lunch and returned to find it off, and it refused to start. By refuse I mean I pressed the power button, the fans (cpu and case) twitched with a microsecond of life then now power. Pressing the power button a second time would NOT trigger any more twitches, however if I remove the power cable and wait for the motherboard to make that almost inaudible sigh (signifying that all the power has been released) then plug the cord back in, press the button and the fans would twitch once more. I thought the power supply was dead. The next day I woke up and the computer turned on like nothing happened, the bios needed resetting but other than that it worked fine.
Fast forward a few months of the computer running fine (slightly overclocked via the bios but I've also tried underclocking it to no avail), and the computer turns off in the middle of viewing a website. It then shows the same symptoms as before. The next day the computer is fine again. One time this happens I test the psu by getting another psu (which doesn't have all of the necessary connections, it's lacking some of the 4 pin connectors for the video card). So I have my old psu hooked up to the motherboard but the new (possibly faulty) psu connected to the video card and jumpstarted so it was on. This powered on the computer but after a while two pops/flashes/bangs happened on the motherboard. I quickly unhooked everything, and sheepishly tried the new psu again and to my surprise everything turned on and ran perfectly...
This happened a few times, the computer decides to turn itself off and I try different things like swapping out ram sticks using only one then trying it in different slots, reseating the cpu, reseating the video card, reapplying thermal paste. All seem to do the trick for a short time but then the computer does it again. Interestingly the bios reports my cpu fan as going too slow like 200-300rpm, so I disable the fan monitor/warning in the bios because I can obviously see/hear the fan going much faster.
Fast forward to this week (May 2012), the computer randomly turns itself off, but tries to power back on (the front case power light comes on, and off then repeats this maybe 6 times before staying off) So I unplug the power cord, plug it back in and then the power goes to the bios. It has a msg about detecting a power surge. The computer is working fine now, but I'm planning on replacing either the psu or the motherboard, then if one doesn't work replacing the other also (basically building a new pc bit by bit until the problem ceases). The only problem is I have no way of reproducing the problem, it appears to be random.
So this is where you come in, what's your opinion or what tests might you try before replacing parts? Which parts do you think could be the culpret? (right now I'm thinking either psu/motherboard, but potentially also the ram, or maybe even actual power surges in my house?).
Thank you for any help or insights you can provide, it's really appreciated.