URGENT HELP PLEASE!

Kia Ora, guys.

I need some help here in kiwiland, well, more like frustrationland.

My pc is a homebuilt one and it has started to turn itself off. This happens in the middle of gaming or emailing, or simply for no reason at all.

I am then unable to turn it back on and i have to turn off the power at the wall for some time, then try again.

Sometimes, i get a bootscreen error showing an overclocking error, even though i keep things as standard.

I hhave replaced the case in case something was shortng out, and have replaced the PSU, but still no luck.

I am gettng some new RAM this week, but when i ran a memory test, i couldnt see any problems.

Things dont seem to be so bad when i reinstall windows, but then the problems still come back

My motherboard has been replaced by Asus 3 times in the last year.

I have an Asus p5b-e mobo, Radeon x550 graphics card, E6420 cpu and 1gb ddr2-800 RAM.

I just bought online the following RAM, but i am scared that it wont work cause only the 667mhz of this one ( 800 ) is listed in the QVL.

2GB RAM

240 pin - For Desktop PC

DDR2 800 - Backward compatible with 533

Non-ECC , Unbuffered, CL5


Can ANYONE help me or give me some idea of what the problem could be?

CPU temp hovers around 40-47 degrees.

I also cant install Asus AIsuite. I have an error dump, but not on me right now.