The cable modem has a network interface. It's not a swappable card like the one in the computer. It could be that interface. It could be your NIC. It could be the cable.
The most likely point of failure is the cable, specifically the connectors on each end.
The second most likely point of failure is the NIC in the computer. Issues could include power management causing the NIC to shut down, a bad transceiver, or a failure to automatically negotiate a connection (10 or 100, and full or half-duplex). Check your power management settings to prevent the NIC from shutting down, and try specifying the connection speed and mode.
Last on the list would be the cable modem's network interface. Coincidently one of the issues there could be a failure to automatically negotiate a speed and mode, and specifying one at the NIC could resolve the problem.
I believe the transceiver on the Linksys modem should support
100/Full-Duplex, but since the DOCSIS standard is only 10/Half- Duplex it shouldn't matter what speed and mode is selected, as long as there is agreement between the interfaces. You can't change what the network interface in the cable modem wants to connect at, so it's just a matter of matching it. And if you have to do that manually instead of automatically, it's no big deal. Try 100/Full first, then 10/Full, then
Thanks everyone. Acting on your tips I discovered the following.
Weeks ago I had taken my long CAT 5 Ethernet cable and coiled it before applying a twist tie. Well the coiled cable (between NIC and cable modem) dangled in the air behind my PC. This apparently caused stress on the connection at the jack (at either end) causing a poor connection.
It is somewhere between the NIC and the cable modem. NOTE: I said BETWEEN them. I suspect you've got a loose connector on one end or the other. Or maybe your pets have been chewing on your ethernet cable. :) -Dave