Originally Posted by demolition
I agree that it is a bad connection somewhere, but the problem does not occur without the ACA020 connected, so I'm assuming it is not in the motherboard?
The bad connection might still be between the 68k and the CPU socket causing these failures?
The ACA620 pushes on the sides of the 68000 pins, which bends them in slightly. If there is a cold solderjoint (which I also believe is what you're observing), the effects will be stronger with the accelerator installed.
I'm a bit less gentle than kipper2k when looking for such a bad solderjoint: I connect PSU and monitor only, then take the board in both hands and twist/bend it fairly hard. If the computer continues to run, everything is fine. If it crashes, I need to re-solder the PLCC chips like kipper2k described: Apply a bit of flux all around, melt the solder for 2 seconds on each pin and usually the computer is immune against hard flexing/twisting after that.
The attached picture was taken with quite a bit of trickery with light¯o lens, so the crack is clearly seen as a shadow. Believe it or not, this A600 was working without an accelerator - exactly your case.