I know Comcast will provide almost endless support if its their modem/hardware, and will more or less snub you if you don't use their modem/hardware (or if you don't use Windows/IE, etc)
It sounds like you either have a damaged line somewhere in the house causing interference, or the signal itself just has really low gain from the cable terminal to the modem, does the modem give you Rx/Tx power figures?
Over the past month or so I've been having problems with my cable internet connection. The first time my ISP sent out a tech (after, oh, 5 calls?), I was told that it was an easy fix -- but the line specialist was going to have to come. I have no idea if they ever came, but the connection improved for the span of about 2 weeks. About a week ago, the problem resumed (slow connection speeds/game lag...followed by total loss of internet connection, which can last from 10 minutes to 3 hours), and now it's gone from a 2-3 times weekly occurrence to every day.
After many annoyed phone calls, they finally sent a second tech out -- and I was told that my problem was due to never registering my "new" modem with them. A modem I bought like 6 months ago and had no problems with until recently. *Cough*. The guy wouldn't listen to anything else I had to say about the line probably being damaged or whatnot. I even tried to show him the modem log (it's a Linksys wired cable modem) which shows all kind of interesting "critical priority" messages about T3 time-outs and such. No go.
My ISP is getting on my last nerve and I have no other high-speed internet providers in my area. Does anyone have any previous experience with a situation like this?