Mike gave you the right answer. The Pro 96/296 is prone to front end desensing when attached to a relatively high outdoor antenna. The "bleedover" you're hearing is actually several stations mixing together on one frequency. The test is to punch in your weakest NWS weather channel. Hit the attenuator button. If the signal gets better, you've hit pay dirt. Here's what I've done to solve the problem:
1. Use the best coax you can. At least a good quality of RG-6 or LMR 400 helps.
2. Apply the attenuator to all VHF high band frequencies you have programmed. The desense seems to primarily affect signals between about 145 to 170 MHz. You should see an immediate improvement in the the signal mixing problem without losing any range.
3. If you are in an urban area or live within sight of a cell phone tower or commercial radio tower, you may need an additional filter. You can buy them from places like PAR Electronics or a number of other on-line sources. I use a combination FM notch filter and 30 MHz high pass filter. The model number is HPN-30118 and I think I got it from Universal Radio. PAR generally has a better selection of filters and they have a good reputation but the price is also higher and this filter has done the job for me.
I have my Pro-96 hooked up to a Radio Shack 20-176 ground plane mounted in the attic, which puts it about 20 feet about the ground. I have a pre-amp attached to the antenna right at the antenna connection with the RG-6 coax running down the walls to the radio in my den. The filter has BNC connectors and sits on top of the 96, one end attached to the radio and the other attached to the termination of the coax. All my VHF-hi frequencies are attenuated. I can regularly hear counties on VHF in the area around Atlanta, which is about 115 air miles from me. I rarely have any signal mixing or bleedover problems.
If you take the steps I've outlined, I think you'll be quite pleased with the results
