Dan, I would strongly suggest that you do all the necessary ground mods etc ( as described in the Marvins guide ) as well as checking those other things.

I recently had some voltages on a Bally power supply board that were much lower than they should have been, so I rebuilt it and it turned out that something on the solenoid board was dragging down the voltage on the power supply board. Did the Marvin mods and it fixed the problem. Its possible your problem could be related to this.

If you dont have a spare solenoid board, I can send a modded one over to you as I have a few spares.