[Beemer]

Run a cat5 line directly to your machine from the exterior junction box. This avoids having other devices on the same line used for your Internet. The rule of thumb is within 36 feet from the exterior junction box.
Other devices on your line, answering machines, cordless phones, corded phones, fax machines, etc, create load on the 1 line looping through your house. Older lines leading to your house can create line noise which interferes with your connection rate. Don't use long extension lines for hooking your machine to the wall mounted connection box. 6' max.
If your telephone service provider is also responsible for the cableing in your area and is supplying your Internet connection, E.G. Telus Communications gives me my phone service and I use their ISP services, give them a call. Complain about poor connection to the Net.

Make sure you check out every possibility that could be the fault of your equipment first. You don't want to get a bill if found that it was your equipment.

Here is a link that will help you check everything out. Make sure you do the US Robotics line test.
http://www.56k.com/trouble/links.shtml

Cheers!