I just had a Bally that was having bonus countdown issues. It took a good while to figure it out, but what it ended up being was the bonus unit zero switch. When bonus was at zero, the switch is open. As soon as bonus increases, then the switch closes. The issue was the switch was closed, but just barely. Meaning it looked closed when I examined it with the power off. But as soon as the bonus mechanism fired during the countdown cycle, I believe the mechanical movement caused the switch to open during the cycle. This made the machine logic say bonus was complete. I adjusted that zero switch to have a bit more force to keep the blades closed and the problem was solved. It took me a few hours to go through the whole bonus circuit and verify all the relays and cycles. I was thrilled to figure out the issue.

The point is, check your switch blades and their open gaps and closed force. I'm not saying adjust each one. NO! Look at the schematic and see what correlates and check those switch(es).