Help debug: PC Randomly shuts off, suspect motherboard.

Hi guys,

I built my first PC about 6 weeks ago.

All was well for the first 2 weeks the I got a random BSOD error 0x00000124

They got more and more frequent and I tried alot to fix this, then the PC would just cut out randomly after 10 mins or so without any BSOD.

My specs are:
OS: Windows 7 64 bit
CPU: AMD Athlon X4 651k
MOBO: Gigabyte A55M-DS2 rev 1.1
RAM: 2 x 4GB kingston hyper x
GPU: XFX HD 5450 heatsink 1GB DDR3 VGA DVI HDMI PCI-E Low Profile
PSU: 450w gaming PSU from novatech
Case: EZ Cool mid tower
SSD: Nova 2
HDD: 1TB WD Cavier Green

Steps I did to try and fix:
- Tried to install windows to HDD and removed SSD, same problem.
- Took side of case off and used house fan for cooling
- Tried another set of RAM sticks, ran memtest overnight and found no problems.
- Tried another GPU
- Updated all drivers to the latest (they already were)
- tried 500w antec powersupply

In my mind I have ruled everything out except the motherboard and CPU, I can't swap these for otherparts as I do not have them.

I don't think its the CPU as the PC would not boot if it had a problem from what I have read?

Is there a way to confirm for certain its my motherboard?

The PC is just for working on, I'm a web designer. It only has to run photoshop + aptana 3.

The symptoms are that after 5-10 mins of use the pc just restarts, the screen gets horizontal lines across it for 2 seconds or so and it reboots.

Thanks

Steve