It's possible that all of those mask ROMs are faulty (they seem to fail for a passtime) OR there's a chance that one of the ICs or custom chips which interfaces with those mask ROMs has developed a fault. Either way it won't be an 'easy' repair unless you're familiar with board repairs. Might be a good idea to get an experienced repairer to fix it for you (I could do it but I'm in the UK and you're in the US - shipping would add a lot to the cost). Someone here that's US-based is bound to be able to help - Alex Yeckley at Electron Forge is a good bet.