Just a guess, but with a larger TV you will see pixelating during fast motion easier. TVs that big are designed to be viewed from several feet away so if you stand a foot or two away, it will not look very clear. Not sure anything is wrong with your box.
Still no HDNET in Mont co. I was in Upstate NY visiting family this past weekend and Time Warner up there has added a bunch of new HD channels, pushing their total to around 115. Cooking, DIY, BBC America, Nat Geo Wild, Hub, GoalLine(new ESPN channel), Fox Soccer Plus, Outdoor Network are the newest ones. They went SDV about 2 years ago and can add HD channels at will. They also had a nice pamplet in the mail with the bill clearly explaining their policy for "Digital channel encryption" and it spelled out how SD/HD local channels are not encrypted per policy and can be tuned with a QAM tuner. It also talked about what SDV is, how they use it, how you can get a tuning adapter, how/what you get with CableCARD VS box, etc. It would be really nice if Comcast put that information in a concise pamplet like this and made it readily available.