i had this same exact problem with my setup. Turned out to be the PPC card, there was a pin on the RAM connector that had bent over. But the machine is seriously unstable until its warmed completely up. I still havent ironed out this issue yet, because the 68k card is rock solid with no stability issues. only the PPC card.

the PPC card does tax the power supply on the logic board alot more than the 68k chip, so if there are any weak caps at all, the PPC will drag it down into the unstable range.