Weird behaviour, random crashes

This is a repost of what I posted in Memory area (found out that this thread is a better place)

My system is 2 years old and is home built.

So, some weeks ago my system would sometimes not boot anymore. Fans would spin up, but nothing happens (no CMIOS screen, no beeps, nothing)
Hard reboots would solve the problem.
Some time later, system crashed while ripping a video (cpu intensive) and it would never restart again.

From what I experienced (fans not spinning up to 100%, leds lighting only for 50% from time to time) I was almost sure it was my power supply.

I got a new one, and the system seems to be working fine.
But ... odd things would start happening.

While booting Windows XP, I would get an error that my sound card was not installed and I had to reinstall (as in: sound card physically not there)
Some other time, a random background process crashes.

I usually play games, and I can say that almost all games I play get random crashes. Sometimes with a popup box saying 'error in xxx.exe', sometimes system freeze, sometimes Blue Screen of Death with imminent system shutdown.

What I did try was the windows memory test at startup (no problems found).
I switched memory from one bank to another, have them dual channeled, nothing seems to change anything.
I tried down-clocking the cpu speed, no change in behaviour.
I dont think its my video card (GF8800GTX) since I dont see weird graphical behaviour, fans are working fine on it.


Does this sound familiar to anyone? Is this a motherboard problem? or cpu problem?
or anything else I'm overlooking?

I could of course buy a new mb+cpu+memory, but that's not my idea of 'fixing' a problem.