need help with muffled radio....

I have a 02 coupe that lost the high freq. sound out of the radio...really sounds muffled, might only be getting base..played with base, treble, fade, balance,etc. with no luck...while driving to my dealer to get annual inspection it started working again fine...mentioned to problem to dealer but they said they did not find any problem. Later on way home the radio went back to being muffled again...anyone have any ideas that might have had the same problem...Thanks. D