Pure DAB Highway radio

Does anyone have the same problem as me. I have the above DAB for the car, to listen to Planet Rock mainly. With the aerial provided the reception, at first seemed to be very good, not anymore. As the system is coming up to its first birthday (christmas present last year) I am having to re-scan the sytem more and more to get a decent signal. Every time it scans, the new frequency it lock onto always seems close to another station and you tend to get 2 stations coming through, not always but some of the time.

Now do I keep re-scanning to get a better signal or would you go for the aerial upgrade and is the upgrade any good, ie:does it work, would love to know from someone who has it.

Miggs.