I don't think I can use my scope or logic probe due to access since the video board is underneath. I'd have to build a harness and get ribbon cables to get this onto my bench ... or even just ribbon cables I suppose to work out of the back of the game. As such, I took the easier path and tried piggybacking which I know doesn't always work. No luck.
I piggybacked multiples at once to save me some disassembly/assembly time. I did ICs 1-6, 82-84, 88-90, 93, 96, 109, 112, 115. I don't have the ICs to try 85-87 (LS399) or 98 (LS241).
Though the graphic glitches are sporadic and don't seem repeatable, one thing that has always been constant is a line on this screen right in the middle. Notice how the center lines painted in the road have almost no gap and the red squares have a white line? The white line cuts right through there every time without fail. My computer background tells me repeatable errors are far more valuable for troubleshooting than random ones. Does that help at all?
If not, I may have taken this about as far as I can without a bench setup. Not sure I want to do that at this point.