Is it my Motherboard??

You didnt give us much to work with... ie. Motherboard model, CPU, memory, video card, PSU, HDDs, Case

yeh ebay not first choice for hardware shopping.... You didnt say whether it was "used" or not.... Try running it with some memory that you know is "good". Id never even consider buying a "used" component unless from someone i know etc.

What power supply do you have in your machine? Make/model? Power ratings?

For a sporadic problem like yours, I would start over. Take out the board, and reconnect all power supply leads and remount the board and cards. It could be a short somewhere or maybe a leftover standoff in the wrong place.

Ok so here's where I am now.

I was on the comp for about an hour with no probs at all then I ran Memtest as requested and, yep, you guessed it, the blue screen of death came calling. So, I thought, aha! its got to be the ram! I then removed the ram and replaced it with a single stick of Samsung 512mb ram and.....nothing, nada, bugger all, wouldnt even boot to splash screen. All seems a little familiar. I changed the graphics card from the top PCI slot to the bottom one and still nothing. Waited about 10 mins and tried again and this time it came back to life. Beacause I had moved the graphics card it then said I had to restart the computer, guess what?? yep you gussed it, didnt even get to the restart stage before Mr Blue Screen reared its ugly head AGAIN!! thats where it is now, sleeping in a quiet corner waiting for me to dare go wake it up again. Lucky I have 2 comps and this is a bit old and not of high spec so really need the other one. Is it time to give up and change the motherboard???

HELP!!!!!

Ok will clear the CMOS. Silly question though…..if I am having a ram problem would it not be the case that the comp would not boot at all? Why does it run perfect for a period of time and then just stop? As you can see I am a little new to this so a bit of advice would be gratefully appreciated. Thanks

with the kind of config u've got dont u think 500watts is a bit too less for ur needs??
get a better psu rated over 600watts from some good brand like antec,corsair,coolermaster.
as for ur bsods change ur mobo battery and even better flash ur bios

Here is my setup and my power supply spec.
GA-P35-DQ6
E4400 @ 3.2~Ghz 9x 355
2 Sticks of 1Gb Ballistix Tracer DDR2 1066 @ 1180~Mhz = 3.33
2 Western Digital 160Gb 16Mb Cache drives in RAID0
ASUS EAX1900XTX @ 702/837 Flashed
Philips SATA Burner
Antec 900 case big heatsinks and fans
My power supply is a ZUMAX 550w with 38A combined rails and has no problems powering all that.
Check online for a good wattage calculator. If you are going SLI or Crossfire then yes get a 700W but, if not 500W will work. I got my 550W because it was a good spec Power supply with the rail ratings and all.

You can trouble shoot it by taking everything out of it and starting with the basics, only the stuff you need like, Motherboard/Processor/Power Supply/Screw Driver. You are going to have to know the beeps and what they mean and have a beeper hooked up to it if one isn't built into it already. Then you flick it on see what it bitches about and go from there. The screwdriver is to ground out the power on button. Don't use a monitor at first just, what you need to start it up. If you can turn it on and off just fine without the memory in it even though it will beep at you for the memory and the video card and the mouse because you won't have it installed/plugged in whatever if, you can do that turn it on and off without any issues with out the memory then you have found your issue probably""". I would start plugging stuff back in and see how far you can get with out it repeating the whole I'm now gonna work crap again. One piece of hardware at a time. You need to know what beeps it is giving you as you go though so, you can plug in what it is asking for as you go. I hope you get my drift here. You might have a bad power button. I have seen it plenty of times. You might just not be using the right memory. If you do these steps carefully and pay attention to how I said to do it you should be able to figure out what the issue is. If it is a bad motherboard then ship it back DOA to the manif. if you can I would think you would be able to though.

I have substituted the 8800 for a 7300 with the same result! The confusing thing is why does the comp run, sometimes for about an hour with no problems at all, then bang, off it goes, dont even boot at all. Then leave it for about 20mins and it all works again for another short period of time. I have swapped memory and everything else I have been advised to do but my sneaky suspision is that its the Ebay motherboard that is breaking down. The ram I am using is compatible with the Motherboard and I had it in another comp with no hassle at all. If it was the ram would the comp run perfect for an hour then just die??

OK well I guess my source was wrong and your psu has 20amps on the 12v rail. However, I still dont belive that is enough to run your system. Also for your question about why it will work for 20min and then shut off. It could be that the psu is able to run your system fine untill you do something cpu or gpu intensive which puts more load on the psu causing it to crash.

Although as you said you also tried the 7300 so it could just be the board. But regardless if it is the board, you will still most likely need a new psu for the 8800.

Something that has not been mentioned yet is the cpu temperature, did you use a quality thermal grease, was the hsk & cpu throughly clean iysophol alcohol, did you put sufficient on though not to much & evenly spread with a plastic card not fingers.
Try & find out cpu temp I use PC Wizard 2008 a freebie do a Google search for it.

Ensure all plugin connections are clean.

Keep Smiling & trying

I told him what to do earlier post. Do a diagnosis test on it and see where it starts screwing up. I am in college for this stuff and already had a decent amount of knowledge before college and have a custom built pimped out machine. I do know what I am talking about you know. My guess is that it is the mother board but, be safe and test everything from the ground up. It can only take maybe an hour to do.