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Q: I have DSL service with BellSouth. It works fine during the day, but at night, usually from about 7 to 11:30, the connection is so weak that e-mail won't download and I cannot connect using my browser. The connection tries to connect, even showing the number of e-mails for each provider, but it won't download.

I've spent hours on the phone with BellSouth, and we've run the same tests over and over (since I always talk with a different person each time I call). Apparently it is something that interferes with the signal during those hours. It also happens sometimes during weekend days and holidays, leading me to believe it is something that someone else is using, such as a shortwave radio, during the time that person is home from work.

This happens every night, not just periodically! I am retired, so I use the Internet often during the day with no problem. But at night, when I really need to access the Internet at high speeds, I am stuck with the slow speeds using my dial-up modem. What could cause this?

— Carol Tapp, Kennesaw, Ga.

A: You did a good job describing your problem and even diagnosing it. No, I don't think it's a shortwave transmitter (and I'll tell you why later). But, yes, I do think you're right on track when you guess that the cause is "something that someone else is using."

My first guess — and I've actually seen this specific problem with a friend's DSL service — is that a lighting system that kicks on automatically at your house or a neighbor's house is causing interference. Or it could be a defective lighting system that is being switched on manually at night.

A system that is working properly shouldn't cause any problems, but if there is a malfunctioning unit, it could be causing the electrical interference you describe. The consistency of your problem — it happens every night — makes me almost sure that it is some device or appliance that is only turned on at night. It's that consistency that tells me a shortwave transmitter is not causing the problem — it's very unlikely that the transmitter would be used every night and only at those times.

It could also be a single light, or a defective appliance, in your house or a neighbor's house that is only turned on at night. And if the light has a dimmer switch — famous for causing electrical interference — it is an even more likely candidate.

Frankly, I think BellSouth should help you find the problem. After all, you're paying for the service 24 hours a day. And I'm sure BellSouth expects to be paid the full amount of your bill each month. So my first advice is to try BellSouth again. If you get no satisfaction from the first person you talk to, ask to speak to that person's boss. Keep going up the chain of command.

Meanwhile, here's a way you can try to find the cause of the interference yourself. Get a battery-powered radio. Tune it to the AM band and find a frequency that isn't being used. Walk around your house, then your yard. Listen to the background level of static. If the static peaks when you are near some object, like a light pole or dimmer switch, that's the culprit.

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