I have my game set up to give a lot of credits so i don't have to add quarters that often. You'll have to check the dip switches on the mpu to see how many credits per quarter it's set up to give. There's a chart in the manual that shows the switch assignments. It might just be set up to give a bunch of credits at once, which will cause the credit sound to repeat many times repeatedly--that would be accompanied by a weird sound, I'd imagine.

As far as the bonus problem, have you verified the shooter lane switch is registering? It's a few inches above the plunger, should come up 04 on the switch test. I think that switch is what ultimately scores that bonus if you manage to shoot the ball back to the shooter lane.

As far as the memory problem, it sounds like you have the original board installed. Is that the case? If so, is there any battery corrosion? It's not uncommon for the ram to get messed up when that happens.