Does standard cable (installed by Comcast) adversely impact an HD signal? It seems like that if I move the cable cord (i.e. I pick it up and move my arm around), it impacts the quality of the picture. This seems strange b/c the connections are all tight despite this. I didn't realize cable cord was sensitive to shaking the cable gently. Are there cables that can be purchased at an electronics store that are better or are they all just about the same? Has anyone ever heard of this problem?
Sounds like you have a bad contact or just a bad wire, call Comcast and have them send a tech out to correct the problem. Yes you can purchase wires at any electronic store, but I'm partial to the cable wires we get where I work. RG6 is a must, stay away from copper-braid and plastic center conductor wires. If you can get your hands on some quad-shield RG6 with connectors already on it, that's your best bet.