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 have an FMC 5800, i ordered some of the chips on the board, mainly the ones under the protective coating, i will try to replace those and see what happens, i will let yall know!
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.