Sounds like you've done some very good work! Based on your first couple sentences I'd also suspect a RAM problem, but just to rule out some quirk with bad disks or whatever on your games, if you boot to a Workbench disk is the machine stable? I.e., if Workbench runs for hours and only the games crash, it might be something else.
Unfortunately I don't know of a good memory testing program, but am sure someone else will come along soon who does, LOL.
