Video artifact finder and all three video memory tests passed with 0 errors.
Memory swapped to original factory RAM again in the only slot I had not tested by itself. Still issues remain. RAM likely ruled out, display card likely ruled out, CPU likely ruled out, hard drive likely ruled out. That leaves the power supply and the CPU, and I doubt it is the power supply. I can put the battery in to check, though. That's the last thing I have left to test.
I'm leaning toward a hard drive related problem through the motherboard. A new hard drive fixed it for a time. Running HD Tune fixes it during the error check. Strangest hardware problem I've ever encountered with a PC...
Edit: Battery did not resolve the problem...