After this many years I think they are probably worn out. Mine were loose and would not make uniform contact even after cleaning.
I purchased all new slide potentiometers from Cessna through Lane Aviation in Columbus, Ohio. The price was quite reasonable (~ $10 each). I considered purchasing equivalent parts from one manufacturer, CTS Electronic Components, but the price was not that much lower.
The Cessna P/N is CM3535-2 and Lane’s number is 1-800-848-6263 if you care to check with them.
One part that I couldn’t find was the rubber tip to fit over the actuating lever – if anyone finds a source for those I would like to know.