Nice call on the 157s. When I first got the board I noted that the legs of most the chips on the VRAM board were corroded. I was concerned that might affect its performace - so I removed them all and socketed them. It was more an excuse to practice with my new desoldering tool than anything.
I placed each chip back where I got it from, so when I fired it back up afterward there was no change in symptoms.
I verified continuity from board to board, and it was okay. No gouges anywhere.
Swapped two of the 157s for each other with no change, swapped another two, and /presto/ there was still garbage on the top and bottom, but it was completely different. I took a 157 from a junk board, popped it in, and all the problems disappeared, including the disappearing Pac-Man and Ghost Monsters.
Thank you all!