Hi, I will start with my computer specs.

MB Gigabyte P35-S3
Intel Core 2 Duo E6420 BOX, 2x2.13GHz, 1066MHz, 4MB
Gigabyte GF 8600GT, 256MB
2xDDR2 1GB PC8500 (1066MHz), Kingmax
Thermaltake power supply Toughpower 700W
HDD Seagate ST3320620AS, 320GB 16MB

When I bought the components and put it all together it worked perfectly for 2 weeks and than it did what seemed to me at a time a random reset, except that I found out later it wasn't a reset but a shutdown followed by a self powerup only miliseconds later. At the time it didn't trouble me much but later that day approximately 6 hours later it did it again, and than it started doing it more and more frequently that evening, sometimes with a few seconds before it powered up again. Than I decided to give it a rest til the morning. The other day I turned it on and it was working fine for 2 hours and than it did it again, and again, and again until it finally didn't power up on it's own. I pushed the power button but it kept happening until 1-2 hours later I couldn't even turn it on again. It was completely dead. Thad day I excluded the possibility of a software problem or virus cause it was happening during playing games, in windows, while loading windows, even in BIOS or few seconds after powerup. I also excluded the CPU or MB overheating cause the temperatures were normal. I decided to take it back to the computer store for them to have a look at it.

The other day before I took it to the store I could again power it up. It wasn't totally dead but it was still doing random shutdowns. After a few days at the computer store they said that GPU died and that one of my RAMs was faulty also. So they said to me they replaced it all and tested the machine and that now it all works fine. I took the machine home and started it. It did a shutdown like 1 second after I pressed the power button and than it worked fine for 6 hours. I shut it down, and went to sleep all happy. But I should have known better. Next day I turned it on and the problems started happening all over again. At first frequently, but now I have a hour or 2 before it does his shutdown. It is still random though. Tried plugging the power cable into another power outlet in the wall. That didn't help either. So either they didn't fix anything at the store, or it broke again.

Anyways, I still have no clue about which component is causing this, and I would appreciate any input you might give. To me it looks like a MoBo problem, but I'm no expert. Thanks in advance

Boot your computer in safe mode and check what will happen.

Try another power supply unit.

see above post.