I had an interesting problem where the signal levels coming into my house were way too high, the cablemodem was reporting a low S/N ratio, and the analog TV pictures looked noisy.
As far as I could determine an amp somewhere was being overdriven. I called Comcast and they sent someone out who confirmed that the signal levels were way too high and that the problem had to be referred to a line tech.
It took Comcast only two weeks to fix the problem, during which time the cablemodem pretty much didn't work. I called up Comcast to complain about the situation and they basically gave me a $240 discount on my bill ($20 over 12 months).
This was about 2 years ago. I sure hope their response time is a little bit better now that they're in the phone business.
