When sharing with another machine, the network connections are much more difficult. I'm assuming you didn't share the modem connection, so that might explain the difference.

The easiest way to check would be to connect the cablemodem directly to your machine, with a new Apple Menu->Location setting. Try that and see if the situation improves. If not, then check with your ISP that they allow filesharing through the Gnutella network on port 6346.

Cheers