Can you access the cable modem and run pings from it? I'd say if you could ping your PC from the cable modem but can't ping the head end equipment than it is definitely their problem. You could also run extended ping tests (go to a command prompt, type ping -t
www.yahoo.com, hit CTRL+C after 15 minutes or so (the longer the better), and see if there is any heavy packet loss)...that might help them.
If you still have a land line, you could get an analog modem and pay for a cheap dialup connection. I used to have problems with my DSL and when I would get disconnected, I'd fire up the analog connection and keep playing.