It could well be an overheating problem. Whenever I tried to play Guild Wars on my old computer, it would always crash (and like yours it could be five minutes in or two hours). I'm not certain about what the problem was, but I think my graphics card overheating was what caused it.

Do you have any older graphics cards that you can put into the computer to try? I found that swapping out the 8800GTS (works fine in this rig, strangely enough) for my old 7600GT worked a charm, and often got me an extra three or four hours of play time.