Computer Restarts without any error, is it PSU ?

Hello people,
A few weeks ago I had an overheating problem with CPU(Pentium D 3.4 Ghz). The temp was like 110C, working a few minutes and than restarting. I knew its CPU problem so I changed it, bought a new Intel Core 2 Duo E8400.
Now this one is working good (temp. 35-42C) but the problem is that after around 8 hours of working it keeps restarting again and the temps are still cool (35-42C).

I made some tests, formated disk and installed a new copy of windows, switched the RAMs in different slots, replaced with different one, stoped the "Automatically Restart" option of windows and still the same problem.
Does anyone know how to fix this ? Is it power supply making troubles?

Running:

Asus P5B-Deluxe motherboard
Intel Core 2 Duo E8400 3.0GHz cpu
3x1GB Corsair XMS2 DDR2 800MHz 1.9v memory
GeForce 6600 graphic card
ASUS Triton 77 92mm CPU Cooler
RaptoxX 550W power supply
Samsung 250GB hard disk
and 2x512MB Kingston memory for testing.

Ok I did that, now what should happent :$?

Thnx for response.

Smoothy1 -

In the past, I've had a case that had a faulty power switch; it would complete the connection on its own, triggering the power-up/power-down sequences on its own. When I went to control panel and made the power switch do nothing when pressed, the shut-downs stopped... But all of a sudden the computer would POWER-OFF (no shutdown-sequence) because the connection had been completed for 5 seconds (equivalent to holding down the power button).

I'm just trying to make sure you're not going to buy a power supply for a problem that is your power switch.