Hello Hugh,

H>>However as the pot is adjusted, the solenoid seems to open and shut erratically, and it should remain at a constant position. Both motor and solenoid have suitable flyback diodes installed.<<

I agree with Steve... I place my bet on hardware...this pot you are talking about... Can you verify it is a linear pot that is not erratic itself to some degree? Dirt etc will cause erraticness. Can you post a schematic?

Dwayne