Need some advice please peeps.

Car radio cassette has been working fine since buying it a few weeks back. On the way home last night the FM radio went really quiet  - I had to turn it up a lot just to hear it, and it was in mono (it had occasional bursts of "normal" volume, which nearly blew thre speakers!).

Same this morning - started off fine then the same happened.

Here's the thing though - it only does it when tuned to a station. When off station, the volume is normal - only static, of course, but normal volume static.

Any ideas?  It's a Sony, btw.