Are you installing any special drivers for the video card? If so, you might want to see how the system fares with just the stock Windows drivers. And if you're NOT installing special drivers, you might want to try the opposite - get the latest production drivers from the Nvidia site and see how they work.
We have tried a couple of things. First the Stock drivers would have been installed from the box, then we tried installing latest Drivers from Nvidia for 64 bit Windows. We'll try the RAM first, any ideas where to get memtest86+ from?