Also have garbage on my Stargate Gottlieb System 3. After change all components and chips on DMD card, sellers tells me to clean ribbon cable. I already did it quickly with contact spray. So I did it again by scratching any ribbon cable and now it works a little bit better so I will change any ribbon cables...
(Same garbage on old Gameboy games who don't works for years...put contact spray but nothing better..scratch contact and all works perfect now