Here are some things to try:
1) Boot from a floppy -- does that work?
2) Check power supply voltages.
3) Although swapping boards should've cleared PRAM, perhaps it's not a bad thing to try (cmd-option-P-R at startup; wait for three consecutive bongs).
4) Check pram battery. If it's sort of dead, that can cause worse trouble than if it were totally dead, IIRC (different models behave differently, hence the uncertainty; my memory degrades with each passing day).
1) Boot from a floppy -- does that work?
2) Check power supply voltages.
3) Although swapping boards should've cleared PRAM, perhaps it's not a bad thing to try (cmd-option-P-R at startup; wait for three consecutive bongs).
4) Check pram battery. If it's sort of dead, that can cause worse trouble than if it were totally dead, IIRC (different models behave differently, hence the uncertainty; my memory degrades with each passing day).