On my first DAB300 S the F connector come loose and started doing that - Richer Sounds replaced it under warranty - it's not got a solid fixing - only appears to be connected via solder so if it's knocked (or in my case heavy coax cable pulled it out of line) the solder could fracture - wobbling the connector around will get a sweat spot.
Go to
www.ukdigitalradio.com and put your postcode in - if you get any green stations then you are in a good signal area....
Do you have a TV aerial? If so get an adaptor from Maplin (£1.70 -
http://www.maplin.co.uk/Module.aspx?...29523&doy=31m5 ) and jobs a good un....
If that works but some stations burble you may need a better aerial...
However it may be a problem I had - it tunes to stations on the lowest frequency by default so a distant multiplex may have added a new transmitter nearer his home and an autotune has screwed it up a bit....