Erratic behaviour with desperation

Hello,

My system is a mistery to solve. Please, if you would be so kind as to shed a light on this matter, I'd be glad.

Specs:

Motherboard: Gigabyte GA-Ep45-ud3l
cpu: core 2 duo E6600 2.4
ram: 2x kingston 667 ddr2 1gb
Graphics: ATI radeon HD 4850 HIS Iceq4
psu: antec 620 hcg (brand new)
case: antec 1100 (brand new)

Problem is this system shuts down all of a sudden, without any warnings whatsoever.

What I've tried already:

*cleaned everything, put on a coat of contact cleaner on everything
*reapplied cpu thermal paste, reinstalled heatsink.
*checked cables, switched sata cable
*switched CMOS battery, cleared CMOS
*tried different configurations of ram sticks
*with HW monitor on, checked temps they are fine cpu is 38ºc, videocard is 48ºC


The story behind this: A couple of months ago, after what I will call a bad overclock, the system simply didn't turn on again. At that time, i used to have a cheap psu, I took the videocard to maximum settings on ATI overdrive (which, as far as i know, is a very secure oc), did everything by the book, and it was ok for two days, games were running flawlessly, beautifully, and then, after a sudden restart, it didn't turn on again. I suspect it was the psu ( i learned after that is important for oc'ing my bad =( ), so I took it to a friend of mine, he tested my psu with that thermaltake tester thing, it was a bit fuzzy at the beginning and then it suddenly was appering to be ok again. tried to put it in, didn't work, tried putting his psu in my system, didn't work either,
his videocard on my system, nothing, mine on his, nothing. Anyway, after a couple of months i tried turning it on again and it worked fine with his psu. Apparently, my sticks of ram were the problem, cleaned them, system was working.
Then I bought a new psu (antec 620..), installed everything and after a couple of tries with the ram configurations, the system was ok, I played games and all. Then I tried oc'ing again and a bunch of artifacts showed up and it turned off.
After that the system was behaving erratically, at times it would post, at others wouldn't. Sometimes it would get all the way to windows 7, then turn off all of a sudden, sometimes the phase leds would just blink and the fans would only twitch, so i did all that above.
It's important to let you know that, originally, I had 4 sticks of ram, two 1gb 667 ddr2, two very old 512mb 667 ddr2, all four of them kingston. After much headache I put aside those old 512, and stuck with those two 1gb just to make sure it wasn't some sort of compatibility problem (even though they worked together for like two years, with issues the were solved with only cleaning and coats of contact cleaner).
Then finally, the behaviour of the system became more regular, simply turning off all of a sudden on windows, posting normally, but every now and then it turns off while i'm doing nothing, just browsing the internet. What could be the cause of this?

I checked task manager and cpu usage is oddly spiking.

I tried everything I mentioned above and I still can't figure out what is causing this. Help me, please!