Up and down again today. It managed to run through the night and most of the day, but crashed some time between 8 and 10PM. I've ruled out the OS, so it must be hardware.
Ran a memory check; no problems. Could be overheating. Could be the motherboard (hope not); could be the CPU (not likely); could be the wireless card (strong possibility). Don't think it's the hard drive (too obscure), and it's not the graphics card or CD-ROM drive (the former has worked in a variety of computers for several years without problems; the latter is crap, but can't cause a crash).
Think I'll pull the side off tonight and see how it fares. If it crashes again I'll just have to start pulling cards until it stays up.