The thought has just occurred to me that if the stick is old, the "Pot" might be your problem. Potentiometer's do wear out and start to give perculiar feedback over the travel. You have not indicated that it ever used to be OK, so I assumed (dangerously!) that you were trying to set it up new.
Also, if it is old, consider some mechanical issues such as a slipping pot shaft, again assuming it is a rotary pot.