GP-100 knob

The Roland GP-100 knob used to select sounds has little increments, each increment on the knob calls the next preset in the sequence. That little knob has gone south. Instead of 5, 6, 7, I get 5, 6, 5, 6, etc and it's almost impossible to page through the sounds. 

Does anyone know if that knob would be a 'standard' component easily found :) in an electronics store? 

It may not be the knob. I can still feel the increments as I turn it so it might be something more expensive that's gone wrong inside the unit but I'm hoping a change of knob is the only problem.

Thanks - rev