Thanks, I located the VRAM chips, took them out and cleaned the contacts with alcohol and q-tips.
It definitely changed the behavior, then there was a vertical bar that followed the cursor around.
There were four chips, so I swapped the "bank 1" and "bank 2" chips, and now the display is perfect.
The bad chip is now in the upper address space, which is probably for the NTSC display, which doesn't work anyway.
The interior plastic of the 8500 completely crumbles, which makes me nervous about taking it apart.
But there's a firewire PCI card, and a "Neptune" disk, which transfers at over 30 MB/sec. So the next project
is to find Final Cut Pro 3, and a DV camera, and edit like it's 1998.
Obsolete? These days, that's a compliment.