Louisville, KY
I had this exact same issue with my Tron Pro and it turned out to be the molex connector for the cabinet wiring harness. Being that both are SAM systems, maybe this is your issue? I would try unplugging the cabinet molex connector (the one just laying on the bottom of the cabinet toward the back that connects to the harness of the rest of the cabinet), and replugging it in.
My symptoms were very similar. Game had been on free play for a good month, and then it would never start a game unless I credited it up (even though it was on free play). Reseating the connector fixed the issue.
