I have the non-sat nav version of the radio.
On certain stored stations (e.g. Today FM), the radio switches to another station by itself after approximately 1 minute. This happens every time on those stations.
I think it is part of the RDS. If you have a certain category of station selected (e.g."local") and it finds a better "local" signal, it will retune to that. Try deselecting "af" (auto follow), though this will also stop it picking the best frequency for Radio 2, fo example, while you travel.
I thought AF stood for alternate or alternative frequency. We have a number of stations in scotland whose frequency changes from one coast to the other. If i turn that off reception would be lost during an east to west journey and equally the other way aswell