DSL starts and stops all day every day

Hi everyone - New guy here. Joined because I've got a frustrating issue with my DSL connection from AT&T.

Easiest way to explain it is that the connection starts and stops. I'll open a new page or click on a link, and the system will load about half the page, then it will stop for 10-15-20 seconds, then load some more, maybe stop for another 10-15-20 seconds, then finish. While it's loading data, the "Ethernet" light on the modem will blink, but will stop flashing (i.e. stay lit but not flash) when it pauses. It does this 90% of the time. There are a few times where it works fine, but I have this problem the majority of the time.

This is a relatively new problem. I switched to this service from cable in January and all worked fine initially. My computer at the time died, so I bought an iMac a couple months ago. Everything worked fine with it initially. Then, this problem started to appear. I'm running OSX 10.5 and the standard Mac firewall.

Last bit here: an AT&T tech came out about a month ago and checked the system and actually witnessed the start-stop of the connection. He didn't find any problems but did replace my modem for free. This didn't help at all. I finally called tech support again last night (after ignoring the problem for a while) and they say it must be the computer. But, I have a Dell laptop that experiences the same problem when connected wirelessly to the same connection. My iPhone has problems too.

Long explanation here, but I'm looking for some insight. Many thanks, G in Bham.