I FINALLY got this glitch tracked down! It ended up being a bad 74LS157 at location 10F on the Video Gen III board. It had a weak 3Y driver, causing glitches on the ORWR signal.
It was a really weird bug to track down... when I had the boards unfolded, it would never glitch, but as soon as I screwed the boards back together, it'd start glitching. So I couldn't tell whether it was a grounding issue, or something else. I insulated all the metal and put it back together, and it still failed, ruling out the grounding issue. I figured it was probably either a hairline crack (flexing differently when screwed together), or a VERY slight thermal issue (board getting a bit warmer when screwed together). I tried flexing the board a bit and couldn't reproduce the issue, so I grabbed the hair dryer, and after getting the board really hot, it would glitch quite often (which was nice, because the hardest part was trying different things and waiting for the glitch to occur).
Once I determined there was a thermal problem, I grabbed my hot air tool, hoping I could pinpoint the single chip with the thermal failure (since the hair dryer warms the whole board)... but was never able to track down exactly which chip it was... just the general area of the board (I was moving from chip to chip too quickly I guess).
When I figured out the general area, I tried my logic comparator, but when I clipped it on most of the chips, it caused other graphical problems (pretty common to happen w/ the logic comparator), and I never saw any non-matching outputs.
So, instead I grabbed a contraption I made a while back to clip on each chip and pull the pins up or down by pressing a button. I found that when I clipped it on 10F and I had the button pressed to pull up, it never glitched, even when hot... and when I clicked it to pull down, it glitched more. I wasn't sure whether it was an input to 10F, or output from 10F that was being helped by the pullup... so I began pulling wires off my pullup clip until just pin 9 was remaining. Pin 9 was the critical pin, and being an output from the chip, I figured that had to be the problem... so I replaced the chip, and it's finally FIXED!
While I was in there, I adjusted the pot so I can get up to 22 on the gas pedal. And the one good thing that came from all the testing I had to do is that I'm finally much getting better at the game. I got my high score of just over 74K.

Before this, I'd struggle to get 20K.
Anyway, thanks for the help!
DogP