I used foam engine cleaner on my two knobs. I rubbed the foam on the knobs with my fingers for a few minutes and it removed what ever causes the knobs to gets sticky. I did this sometime ago and they still look nice and shiny, and are no longer sticky. I might add the foam engine cleaner was the cheap stuff I used on my truck. It came from Wal Mart.
My recent edit, I want to say, I did remove the knobs to clean.