Grady,
That doesn’t sound like the microcontroller to me. If it was the microcontroller, you’d have more problems…the other features wouldn’t be working properly….but i may be wrong. To me, it seems like a bad solder joint somewhere. Start by re-flowing the solder joints on the depth knob. But just for good measure check to see what voltage you get on the center pin of the depth pot (or pin 23 of the micro)…you should be getting from 0v to 5v as you turn it up (from counter clockwise). Also make sure the micro is seated in it’s socket very well.