It's occurring on the exact same frequency, with the exact same scanner (RS Pro 404) hooked up to the exact same sound card (Sound Blaster Audigy SE) through the Line In.
Ok. When you say "different frequency" do you mean tone frequency or broadcast frequency? If you are scanning two different broadcast frequencies, what are the two frequencies? It may be that the system is catching a sideband if they are too close together. Also are you close to the broadcast tower?Same thing is happening on the same scanner, different frequency
I would assume that if you are scanning two different broadcast frequencies and they are close together that what is happening is that when the page comes through the scanner keys up on the 'wrong' frequency and the text is garbled. Similar to tuning your fm radio close to the stations true frequency (tuned to 99.3 instead of the proper 99.5) when close to the tower.
Otherwise we will need to look into a short or something further if it is just one frequency.