Hi Jeff,
It is very difficult to say exactly what component is drifting without doing some measurements where it is happening. First assumption is that it is the LO drifting, i'd start with monitoring the local oscillator frequency, pin 6 of HA1197(Q7). Best setup is to use a low capacitance probe to a frequency counter.
If it is that, then you have to figure out what exact component in the LO is drifting. There is not much there, T2,C84, VC and trimmer. It probably is temperature related but the ? is which one is it.
The 1050 and 1980 ckts are much the same, you can compare the components used to see if there is a difference in components used.
You can inspect the var cap for tarnish in the fingers that short the shaft to the base. Have you ever cleaned the var cap in the 1980 to ? You could try to apply some fader lube, very carefully to the grounding fingers and see if that solves the problem.
Rick