On my Fish Tales U14 has a socket, but my machine doesn't have a ROM there. Yours should be on a socket as well. Try pushing on the chip or pulling it out with a small flathead screwdriver where you gently lift it from each side one after the other and putting it back in. Can you inspect the pins to see if they need to be re-soldered ? If a ROM goes bad and fails its checksum test, it will never ever pass it again. It would be real bad luck that this ROM died over night. So if at any moment you boot the game and don't get that checksum error, I would assume the data on the chip is still good, but connections to it fail sometimes. I would try to pull out and re-seat a few times the ribbon cable interconnecting the cards. This could very well be caused by a bad connection somewhere on the data bus and re-seating the ribbon cable could always help.
I've only been learning about pinballs in the past month since I acquired my first pin (Fish Tales). I have been repairing electronics for the past 20 years though. I just tested on my machine and if you pull out the ribbon cable from the DMD and put it back in , the display continues to show you graphics. You say that restarting the machine will have the dmd work for 4-5 games, I would try once the display is showing garbage , to unplug the ribbon cable and plug it back in. Is it still garbage ? If so , I would then try to unplug the ribbon cable first , and then the power connector next (be careful if you do, there's high voltage there). Plug the power connector back, then the ribbon. Still garbage ? Then the DMD is likely not at fault. If unplugging the cables fix the DMD , then the DMD would be to blame. I understand its a "new" one , but you did try it in the machine when it still had high voltage issues I believe? Maybe that new dmd was damaged by the high voltage as well.
If you always have garbage when unplugging the cables from the dmd and only when you restart the entire machine the garbage clears , the problem isn't from the DMD but on one of the boards. You also said there's a new dot matrix controller in there.. Which was also connected to the high voltage ? Then it *could* have been damaged as well..
It sounds like something is affecting the data lines, creating checksum errors and garbage on screen. I am not yet knowledgeable enough with workings of the different boards and how they all interconnect, common issues and etc. Perhaps some of the Gurus around here might have a specific chip in mind to check. You do have the luxury of having more pinballs with similar boards if I'm correct, now that the voltages are fixed I don't think you risk much swapping some boards to isolate which one is causing the issue IF we can't isolate the issue. If you do swap boards, I would swap one at a time, if the issue persists, leave that board there and change another, and keep doing that until you've changed the right boards, then one by one I would put the old ones back and test after each one, just to make sure the issue hasn't affected more than one board. It is a lot of work, but in the end you'd be sure exactly which board is the culprit.