A problem that I have seen with a LOT of older trim-pots is that the wiper has distorted the resistance material at the point where it sat for ages. You try and calibrate the instrument and you find that there is a funky place at that setting of the pot and you can't set it to the desired/required value. Putting a new pot in (or swapping it with another pot with that was at a different setting) is the only cure.