Servo drift occurs when the potentiometer shaft (the one on the left in the photo) turns from the centered position (ie 90 degrees). I tried to prevent servo drift by soldering the pot shaft in place, but apparently all I did was solder it to a piece of metal that rides on the shaft above the pot body. Hence: drifting.
Resoldering didn’t work, so I tried glue this time. It’s cold glue, though, and servo conversion procedures recommend hot glue. Hot glue is a hassle, though. I want to see if cold glue works.
(Photo notes: the glue is the white stuff at the base of the potentiometer shaft. The previous solder is the silvery stuff on top of the metal plate, after efforts to de-solder. The solder which somehow ended up on the shaft easily snapped off. On the right, the grayish goop on the motor gear is a lubricant put there by the factory.)