I'd still like to see a set of voltage readings with 5V on the post and just a voltmeter on the output. A set from either direction would be useful.
One thing you might want to do is loop on the read until the value settles and not just take the first value. I have to believe something is off here, there would be no point in putting detents on the pot if it was that flaky! I'm having a real problem with the voltage varying by a 10:1 ratio between stops on a linear taper pot, that makes no sense to me. If the A/D is getting true values, I would expect maybe a 10-20% variance in the step sizes, but not what you're seeing!
