Is this AM or FM?
If you tune a frequency, the radio automatically fines tunes to the exact frequency. If the signal you're trying to tune is too weak, and there is another signal on the same frequency or close to it, the radio might tune back and forth between the different signals, depending on which one is stronger at the moment.
OR, are you trying to describe your Radio Scanning stations and NOT stopping on the station you want? OR is it somehow initiating scanning to a new frequency on its own? You're being vague and using vague terms, its very hard to tell what your trying to describe.
I can't tell if you are trying to tune in weak radio signals, that are weaker than usual and think that is a problem with your radio, or if your radio is malfunctioning and you're just describing the symptoms poorly.
Have you tried tuning the same stations with another car radio at the same time of day? Is it stronger than the radio in your Commander?
Have you tried tuning in good strong radio stations? Does it still do this?
If the problem is that the radio recieves all signals much weaker then you have confirmed those signals are, I'd suspect the antenna and the coax cable from the antenna to the back of the radio.
If the problem is that your favorite radio station, that is far away, has started to be recieved a lot a weaker than in the past, then complain to Marconni, thats Nature.