I would bet your issue is still a flakey switch. It’s not likely to lose power to just one solenoid if there is a board issue. You could have a poorly soldered ground or power wire, but I assume you have checked that. I once had an opto that tested good on switch edge test but during gameplay it would not function. I replaced it and my problems went away.
Edit: after reading post more thoroughly, if reseating the ribbon cable seems to do the trick, either you have some cold solder joints on those connections, or you need to replace your ribbon cable.