I just tried this again myself as I too had a grumpy A500 rev 5 that would not take a KS3.1 ROM. Mechy's advice is correct, there IS a difference on how the jumpers are done. I tested the usual Pin 1-31 and 31 to 21 mod just now - did not work. So, I then did the one Mechy described above and it works.
Also, on this machine, the Motorola 68000 would not work - ROM would start up and then guru. Replacing the Motorola with an ST68000P8 clone and no problems. Perhaps they have a lower tolerance with the bus noise. The 68HC000s are also much more forgiving.
Anyway, try Mechy's instructions, they worked here just now.
