Thanks for the tips. I've pretty much tried them, but not in a very systematic way: I removed the Supra card, crash; I replaced the Supra then removed the three SIMMs, crash; but I didn't ensure the crash was from the same problem. My plan is to repeat what you suggested, but try to give each step a day or so. But I really want to remove the GVP card, and see if the problems pop up with a stock systems.
Anyhow, I opened up a thread on the hardware forum. It's probably more appropriate.