NAV PLUS RADIO bad reception

I had my A4 2002 with nav plus radio and 6cds, for almost two years and in
that time the radio reception on FM has gotten worse to the point of local
stations being bearly listenable and national stations dissapearing into to
absolute noise. Now, just the local radio station can be syntonised. The
curious thing is some times when i turn off the NAV PLUS and turn it on
again, all the radio stations come back ok again. But just about little
time.
Which will be the problem?

Peter Moura