>The problem described below has been solved, but I don't quite understand it.
>
>This morning, I couldn't connect to the Internet from my computer. I used my children's computer for some emergencies.
>
>When doing a ping, from my computer, to the default gateway (the ADSL router), about half of the tries were successful.
>
>The other computer worked perfectly, so the problem didn't seem to be the ADSL router or switch.
>
>I contacted the person who does tech support at our local Cisco Academy, and he said, more or less: Well, you might try buying a new network card. After all, they are not that expensive. Or perhaps demagnetize the network card.
>
>Following his instructions, I unplugged the computer power cable, and touched the metal contacts with a metal tool that had an insulated handle. (He said "screwdriver", bud I didn't find one at that moment.) That was the connection for the power source, I didn't do anything with the network card.
>
>The whole idea is preposterous, except for one detail: it actually worked. I had seen it work before, on one of the computers at the local Cisco Academy (but I hadn't remembered the details).
>
>Important note: Careful with this - I understand the power supply can have dangerous voltages, even with the power cord unplugged.

Sounds like it might be a grounding problem:

- possibly at the computer
- if your computer's ground is good, maybe the router/switch is not grounded properly, and you're getting a voltage potential between the two ends of the network cable

In either case:

- check the quality of the ground at both computer and router/switch. Actually, most consumer routers/switches use AC/DC power adapters with 2-prong AC plugs that aren't specifically grounded (at least, here in North America); this may or may not apply to you with your router/switch. As for the computer, I remember seeing years ago, recommendations to ground a computer's chassis by running a thin lead from the chassis to the center screw holding in the wall outlet cover plate. Don't know if this is still relevant or valuable.

- check for bad or corroded contacts. Remember, 80% of electrical problems are caused by bad contacts. You could, on-spec and/or for general reliability improvement, disconnect everything that comes apart and wipe down the connectors with contact cleaner. There are also contact-enhancing materials such as Stabilant. FWIW Jerry Pournelle in his Byte Magazine days used to swear by Stabilant.