I also have the same problem with my upgraded ATT DSL service. I was on the 3MB up and 300kbps down. then upgraded to 6MB/s up and 768kbps down. The first couple of days, the transfer speed is excellent in both up and down. Then today, I discover that my upload speed is just really slow only at 100 kbps or so. The download speed is also a bit slow as well at around 4MB/s.
I talked to the ATT guy and they ahve checked the line and speed and said that it's fine.
I spend hours finding the solution. And finally thought about upgrading the firmware. I am still in the 1.00 (thought to myself if it ain't broken, why fix it). Then I though tI will check the forum first. and god I am glad that I did because I find someone else who got the same problem.
I then thought about digging into the router setting and I think I may have found the solution.
In the Advanced and QoS Engine, I found that the "automatic uplink speed" is checked and underneath it, it actually limit the speed to only 120. I disable that check and then manually enter the upload link to 768 and did a quick test. Now I am getting a decent upload speed.
I will hae to do more testing to see if this fix the problem.