UPDATE:
Couldn’t find the cause of the problems. Decided to take it to an experienced tech (Corey Davidson at Davidson Electronics). After tons of frustrating troubleshooting, he narrowed the problem down to the D/A converter. As original AM6012PC chips are quite expensive nowadays (around $60 for NOS or new), I did some research and found out that the DAC312 is supposedly a direct modern replacement. So, the tech is going to install one of those and calibrate it. Hopefully, that solves the problem. We’ll see!
Thanks for all of the help!