OK, I can understand that from using analog scanners and high power paging towers near the frequency I am trying to receive. But what is happening is if the transmission starts out good it remains good. Here in Cleveland we have some long winded dispatchers that go on-and-on. I would think that at some point in their transmission, sometimes more than a minute, I would expect that another interfering frequency, perhaps on the same tower would kick on and cause my reception/decode to deteriorate in the middle but that isn't happening to me. Either the transmission starts out good and remains that way, or it starts bad and remains that way.In my experiences with trunking and scanners, the issue I've found which supports your error rate going all over the place is adjacent channel interference.
Scanners have a hard time blocking out adjacent channels and depending on how your system is set up there may be repeaters on the same tower transmitting occassionally at the same time. For instance here in Detroit, I can see the tower, it's about a mile away. It has frequencies of 851.675 and 851.7375 in service. Needless to say that I can hear one or the other fine, but combine the two and the BER goes through the roof to the point you cannot decode anything, but still have "full signal bars".
Motorola and other professional radios have ultra narrow filters to keep adjacent channels from interfering.