As per AG6K's page on somis.org :

Rapid jumps in VFO frequency. Cause(s): [1] This can be caused by fluctuation in the 9V regulated power-supply voltage, which is the result of an intermittent connection between the AF-AVR [automatic voltage-regulator] Unit's circuit-common and chassis ground. Fix(s): [1] Solder a wire to the "TPG" terminal, next to C81, on the AF-AVR Unit. On the other end of this wire, solder a #6 ground-lug with locking teeth. The ground-lug is placed under one of the nearby sheet-metal-screws that fastens the AF-AVR Unit to the chassis.

An alternate method, if you are already doing other work on the board is to merely scrape off the coating at (2) locations
on the foil side and make solder jumpers. This will effectively bring the floating ground condition permanently to a
hard wired ground coming in on the +40V line for the power transistors. That way if over time the screw loosens, its irrelevant. This photo shows the procedure, works on both AVR boards in either radio.