So you had this issue prior to code change. Does it seem to get worse or better the longer the game is on? Reason in asking as the head box heats up from running lamps,power supply etc. It is possible to have a solder joint or joints that are making and breaking as the temperature swings. So check the header pins on mpu board and display board for cracked solder joints.

Not being a hook owner but having two data east small display games the power supply in these and other de games are always suspect until it has been gone through. They have a bad habit of smoking to burning up and cooking traces off the board. As the dmd gets older and weaker they tend to pull more power and strain the not so hot power supply as others here have stated in past discussions. I would start here 1st because if voltages are not to spec the issues can have you running in circles. The good news is if the display is really going bad we now have a led option to replace it this is fairly recent change as proir to this it was buy a working used one and pray it lasts.

It has also been suggested by others to replace the display ribbon cable after years of pushing and pulling the connectors don't hold and conduct as well as it did back in 1992 when it walked off the factory floor.