I just thought I'd post this in case it helps anyone. I've found a way to force the modem to hang, useful for demonstrating to a techie who doesn't believe there's a problem.
I managed to get a Premium Xbox yesterday, so the first thing I did was to try and download fight night 3 (400+ Mb). I've been using a core for the past few weeks so everything to do with Live is OK, plus I know my way around the network.
Anyway, out of the 20 or so times I tried, the modem hung after 20 seconds - consistently. Didn't matter if it was in one stretch or not, I tried to download for 10 seconds at a time, after the second time the modem hung. Basically, it would only allow a set bandwidth to be downloaded before crashing.
Just to make sure it wasn't something else, I downloaded a different demo - exact same thing happened. The funny thing is, I've calculated that the modem is allowing about 7-8Mb to be downloaded before cutting out. But from my PC I downloaded a 30Mb file with no problems
Something definately dodgy is going on, but hopefully someone might be able to use this information to force their modem to hang as a demonstration of the problem.