Inside the radio there are capacitors, which among other uses is the filter capability. Usually when these start to go bad they will not work, much like the tach filter that will not allow the tach to work until the engine has sufficiently warmed it up.
If you are the least bit electronically inclined you can replace them, Radio Shack usually has a good selection for replacement... If not I could always look at it for you. Drop me an email...
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