The analog circuitry under the protective coating is extremely sensitive to moisture. I went through extensive testing and the solution for me was simply to keep the board indoors and dry between uses rather than in an unheated shop. The previous owner must have realized this because I found a 250 watt light bulb in the cabinet that was keeping everything warm and dry for him.
I did have a flaky rotary encoder position sensor. Seems there are at least 3 versions of position sensors. Mine was upgraded to a rotary encoder around 1998 so it does not use either version of the disc with the position holes.