Get a DMM/Voltemeter to measure power supply logic +5VDC rail. You'll need to measure DC, and likely AC ripple voltage. Computers (including the computer that runs your pin) need stable and precise power supplies. Verifying correct power is step #1. Have the power filtering caps ever been changed while in your possession?

Power supply connectors CN1 and CN2 should also be checked or repinned. Sometimes, but not always, you can verify a cracked or oxidized connector pin by gently wiggling the connectors during gameplay. If it resets then, you've identified your problem.

http://techniek.flipperwinkel.nl/desega/index2.htm#reset

Well, sometimes it is. Provided the connectors are in good shape without damage, cracked solder on the pins or breaking crimps for example - there's not much surface area and a possibility of oxidation. Read on http://www.pinrepair.com/connect/

I had this same thing happen (I've owned 3 Secret Service machines) and it turned out to be the mains transformer (CN1) plug that plugs directly in to the power supply. Replacing the burnt/corroded pins on the wired harness side resolved the reset/non boot issue for me. Pictured here:

Screen Shot 2017-12-13 at 9.20.54 AM (resized).png

A very common Williams/Data East problem.