How does the P25 system come in on just a rubber duck antenna? If it's that close, I'm guessing that you are overloading the scanner's front end by feeding it from the discone up in the air. With a system that close, and considering your other interests, it may not be as simple as switching scanners; you may end up with another scanner for the air band and UHF/VHF, and need to use a band-stop filter to keep the 800 MHz out of it. Do you have any FM broadcast stations fairly close? if so, an FM trap may also be helpful.