I had a game doing this to me. It was freyed right behind the plug. Also had it happen when i had to use an adapter for an ungrounded outlet at my old house.

Two things to check:

Ground prong missing from machine's power cord?

Wall outlet's ground working properly? Use an outlet checker to verify.

Once you have done this, ohm for continuity from the ground pin to the rails/coin door. No continuity on both means you have a break in the safety ground. Ohm out the third pin to the green ground wire at the power entry
If it has no continuity, replace the power cord. If it does, then look for a break from the ground wire to the braid.

Replace the power cord
Must have 3 pin for proper ground

If you read the previous posts, he says the third pin is present. But it must be TESTED - along with the outlet - to be sure that the ground is proper at the wall outlet, making it through this cord, and then being distributed into to the game to the metal components of the chassis. Just blindly replacing the power cord might not help at all if the problem is elsewhere.