Every time the motor stops it spikes the switch matrix. The motor should only turn off at the end of ball, my black pyramid pops the bumpers or slings almost every time the motor turns off when the bonus countdown starts.

Make sure the solenoid expander relay is not fluttering. It may flutter if the load lamp is burned out, not working, or a LED lamp. Check the load lamp works in lamp test and you don't hear the relay flutter during lamp test

Put a cap across the motor leads and a cap from each motor wire to ground. That will help eat the noise. Not sure how big of a cap you need.... I think Stern uses 100nF caps on their DC motors in new games.

Route the motor wiring away from switches. It might be someone impossible to do this. Just pulling out the flipper wires away from the switch matrix wire in the loom helps with the phantom pop bumpers with flippers, probably would help avoid the noise from the motor getting into the switch wiring.