[MaggieL]

The effect you're seeing is called "FM capture", and it simply means an FM reciever will "capture" and reproduce the strongest signal near the frequency it's tuned to. As you drive around, signal strengths vary widely due to multipath reflections off of buildins, bridges, etc. Some FM receivers have "Local/DX" setting, which reduces this "capture" tendency somewhat; but these days that feature isn't seen as often; check the manual for your car stereo.

The only other thing you can do is increase the signal strength as heard by the receiver. Getting the sending antenna closer to (and parallel to) the receiving antenna is pretty much the key. Verbatim's suggestion has some merit, although I doubt you'll want to have wires hanging around the outside of the car. If the sending antenna is a piece of wire, wrapping it around the antenna cable *inside* the car (before it plugs into the stereo) may help, if you can access it behind the dash.

Also make sure the batteries in the sender are fresh (unless it's one of those that just plugs into the cigar lighter). It's also possible that a more expensive unit may have higher transmit power, but difficult to tell without trying it out. .