It has been a year, but I finally got around to fixing this. The problem was the 500k trimmer pot. I bought a new one from this link.
I desoldered the old one and plugged this one in. The new one was slightly bigger, so some minor bending, and it was in. Resoldered, then I had to do some dialing-in with a small screwdriver while it was running. I used Frogger for the fine-tuning.