These Delco radios use a 5-pin digital rotary encoder. There is a tiny bearing ball on a track that touches contacts together when you turn the knob. Eventually, the ball will flat-spot and not move properly. The encoder is made by the Alps Electronics corp., and they will NOT sell you a new one. (I've spent a LOT of time in the past working this one out...) I was working on a modification that changes the rotary knob into a rocker switch that you press up or down to change the volume, but never finished it. The switch was tied internally to the steering wheel interface for easy wiring.
You can also turn your worn-out encoder while pulling out gently on the knob - that seems to make them work better for a while.
I just use the steering wheel controls...
