Generally ( since I am not familiiar with whatever radio you have in this car):
Remove the radio from the dsh
Open the radio casing (probably tiny screws which will immediately jump off the workbench and into the debris on the floor)
Inside of the area behind the volume knob, there will be a shaft from the knob entering the "pot"; may be more than one, if your balance/fader functions are also on that shaft. If by chance you have a separate balance or fader, clean it also.
Spray with electronic contact cleaner,then run the shaft through a full range of functions and full range of motion. Repeat several times.
Let the spray dry off, reassemble radio, plug in and hope this was the problem.

While you're there, check and clean all the external contact points and connectors with fine sandpaper--power, speaker connections, ground,antenna connector, etc.

Most of the radios will have an antenna connection which is a stiff friction fit; grit your teeth and pull if that's what it looks like. Be sure to clean it also before you put it back. If there's room under the dash for it, and the antenna wire is too short for comfort, you can get antenna extenders at Radio Shack for a couple of bucks--can save a lot of profanity and bandaids in some cars.

I know this works on the CR-814, because I've done it. According to others on the board, it works on any radio with these "pots" and the appropriate symptoms.
If it doesn't solve the problem, consider it a necessary step in the differential diagnosis via ruling out one potential source of the complaint.
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We have met the enemy and they is us. [Pogo] S70 T5 cop car : Rough Rider suspension, Walmart fog lights, eBay speakers, ambiance by Pall Mall, trim by Le Duc d'Tape, 8-channel THD by OEM amps