Possibly a thermal problem with the transistor that controls the knocker. Stops functioning when hot. Go to test mode and while a friend is firing the knocker, train a blowdryer on the transistor (CAREFUL:not too close to the circuit board, you don't want to burn the board or get it too hot too fast) and see if the knocker stops firing after the transistor heats up from the blowdryer.

Are the wires secure on the knocker coil in the bottom of the cabinet?

Does the plunger move freely in and out the knocker coil? It may need a new coil sleeve.

Also, don't forget you have coil test mode in the diagnostics. When it is not working, you can put it in test mode and wiggle the wires and connectors and gently tap on the board to verify if there is a cold solder joint.

I always use self test for issues like this. Keep in knocker test and let run. Issue probably loose copper wire on coil itself. Ive seen ops toss cashbox lid in cabinet damaging lugs and or coil over the years.