Darrin:If its NOT a birdie - any chance you are located near some large FM Band station antennas? I had a Pro-2004 that absolutely had to have the incoming signal attenuated when trying to monitor around 155mhz. My $1500 Icom R8500 has the same problem although not nearly to the degree the 2004 suffered from it. All caused by our friendly FM stations just up the road.
There was NO AUDIO...just a very strong signal that given the engineering of the mixers in the receivers killed my 155mhz stuff. If you find that the receiver's attenuator seems to be "working in reverse" with regard to the problem channel then I would guess you have the same issue.
I ended up buying an FM notch filter from OptoElectronics (there are others) and it notches out 88 - 105 mhz....problem is completely gone. I would try the same thing on your 780 if you can't solve it otherwise.
Curt