Codes P0014 and P0306

The car has been great for the past four years that I have had it. Recently she began having a rough idle, I live in New York so it was around the time my inspection was due, the check engine light came on so I took the car to Meineke who has serviced and inspected it in the past, they gave me the codes and explained that one was the camshaft sensor I believe (P0014 code) and the other was something with cylinder six in my engine (P0306 code) they advised I bring it to GM (They produced the Ascender in the U.S. That year, similar to Envoy or the Trailblazer. I took the truck to GM they read the codes, and the tech switches the coil from cylinder six to five, the problem did not follow he did check plugs and stuff and only changed one, they said they think it is a valve and refused to work on it because of the miles, at a dealership. It drives great but idles pretty bad on traffic or In park but drives super good on the highway when moving. Any Thoughts or similar situations?