Welcome to trying to listen so CQPSK on a radio that cannot properly decode it. Until the scanner manufacturers' wake up and start building radios that can properly decode CQPSK modulation, which is used by multisite 800MHz systems like we have, this is what you get.
Trying different antennas, attenuators, etc may improve it but it's just how it is. It all comes down to location and signal strength being dominant from a single site to reduce to BER.
Uniden, GRE...why don't you guys get the clue and work on a unit that can properly receive CQPSK?