Issue: The DMD is showing either all dots lit, or garbage. Sometimes it would show some scenes correctly, but most of the time garbage is displayed.
Solution: Changed the display roms to known good roms, nothing. Decided to remove the custom PALs and the memory ic (a 62256) to give the legs a good cleaning. While removing the memory ic, i noticed two bent (!) legs under the ic - whoever did this certainly never tried it afterwards.
Trying to straighten out the legs broke them off. Cut off two legs from a junk ic, soldered them on and put the ic back into the socket. DMD is now fine. Issue resolved.