Clip a voltmeter to the 5 volt TP and ground TP on the cpu. Play some games and monitor the voltage during mutiball. Make sure you have plenty of gap on the slam switch on the coin door. Report back the voltage you find.

On the CPU board there is a metal stud between U-21 and U-22 that is for checking the 5 volts that is on the board. And just to the left of the battery pack the is another stud marked ground. Its important that these are used for testing. Testing the power supply 5 volt doesn't mean that it actually made it to the CPU board. The CPU board is what is causing the resets so you need to know what the voltage is on the CPU board when it happens. It could be heat related so you need to clip on to the test points and close the insert panel and have someone watch the voltmeter while you play games until it resets. Let me know what the voltage was just prior to the reset.

If it were me I would replace every diode on the coils. This is a cheap and easy thing to do. I also had game resets from a bad U-27 ROM socket, but this is a much harder thing to change and I don't recommend doing it unless you know for sure that its causing a problem. On my board I could apply light pressure on U-27 and it cause CPU reboots.