Why does my internet connection slow down sometimes?

Off the top of my head, here are some reasons you might be seeing those performance characteristics:

• Activity on your own machine. I suspect this is not the case, but I do have to call it out since it could be a biggie. Basically anything else you're doing on your machine that accesses the internet could be adversely impacting the speed tests that you're running. Of particular note are viruses that may have planted a spam-bot on your machine. They'll periodically start sending big batches of spam which could certainly affect your apparent internet speed.

• The quality of your phone line. I ran into this myself some years ago. Turned out I had a noisy phone line, that was impacted by water seeping into the wires underground. Once the telephone company came out, discovered and fixed that, life was good. Even with my more recent T-1 installation, the technicians had to switch the wires used because of the poor signal quality of the originally selected pair.

• ISP Traffic. Even though your DSL connection to your ISP is dedicated only to you, at some point "upstream" it is aggregated with the internet traffic of others. Cable users see this most frequently, since their bandwidth is not dedicated, but rather shared with their immediate neighbors. If there's something happening to use up all resources after your connection is aggregated with others, the impact might well be reduced performance.

• Traffic at "the other end". I'm going to assume that you're seeing a general problem across all of your internet connections, but if it's a problem with one particular site or service, the issue could easily be at their end.

• A poor quality or dying DSL modem. I experienced this as well. Rather than just fail outright, I have seen DSL modems die a slow death, and performance is often one of the early warning signs.

• Problems with your own network card or equipment. This needs to be included for completeness. One of the things that most ISP technical support people will tell you to do is to disconnect everything but one computer from your connection, and test it that way. It's annoying, but there's a reason: this eliminates all of your other equipment, such as routers, switches and other computers, from impacting the results. In your case, it would be interesting to do exactly that, and then see if the result change if you use a different computer.

Unfortunately, as you can see, there are many places to potentially point the finger. A good ISP will help you diagnose at least somewhat, or perhaps put you in contact with the data side of your local telephone company that actually provides the physical connection into your home. (As an aside, even though your ISP and your telephone company might share a name that does not mean that they are the same company, or that even if they are that the two divisions work well together. Often, it's just the opposite.)