Lately we've been seeing an awful lot of hassle with cable modems...

Three things have stood out.

With machines that seem fine but won't browse the net, two culprits have been common:
WinMX - Uninstalling it makes the machine work perfectly, and,
NewDotNet - Running AdAware makes the machine work perfectly.

And the third problem we've been having is with browsers unable to access some high security websites. This has been solved by making sure "Use a proxy server" is NOT checked.

Not saying this will solve all of your cable modem grief, but I've noticed these three trends in our store over the last few weeks, so if it helps anybody in their troubleshooting, all is good...