Riff Man J,
That’s a very interesting problem to have. To me, it seems like the pot may have some dead or dirty spots on the carbon track that’s bringing the voltage down to 0 volts at those points. You can check this with your voltmeter. Test the voltage of the center pin of the pot when the wiper is on those dead spots. The center pin should have a continuous voltage from 0v to 5v as you sweep the pot from one side to the other. If the voltage drops to 0v on those spots then the pot needs to be cleaned or replaced. If the voltage is continuous all the way across the sweep of the pot then the problem has to be software related.
Let me know what you find.
-Brach