Roamio Plus Picture Problem, High SNR

I ordered a refurb Tivo Roamio Plus in May, and I've had problems ever since. I finally received an updated Comcast cable card and the gross errors disappeared, but I've had ongoing issues with several local channels. I'm getting horizontal streaks and audio dropouts on my local ABC, NBC and Fox hi def channels, while the rest of my channels seem good. I had Comcast out to check, they confirmed the problem but insisted the signal was fine. I had them hook up their dvr and tested them side by side, the Comcast picture looked great!
I called Tivo support, and the rep walked me through diagnostic tests and said the problem was my SNR was too high at 38db, should be between 29db and 35db. The Tivo rep said Comcast needs to attenuate the signal, but when I asked them they refused. They told me their spec was 37.5db or above, and lowering the SNR could cause them problems!
I have ordered a 10 pack of 3db attenuators, and I will add as many as needed to get the SNR down to Tivo spec. If that solves the problem fine, but if not I will insist on a replacement Roamio Plus, and if I still have issues I will want a refund. I refuse to get caught in the middle of a pi$$ing match between Tivo and Comcast, I pay a lot for the service and I want a good picture, not excuses.