Interleave mode is allowing you to connect with a Pro profile. Without it, you could probably only connect reliably at Basic speeds.
The bottom line is that your attenuation is really too high for Pro speeds so you will periodically have speed train downs or sync problems. I wonder if your attenuation jumped for some reason after AT&T sold you the Pro speeds? I didn't think that AT&T would sell Pro knowingly with a loop as long as yours.
Those latest modem stats suggest that your low upload speeds may be caused by anomalously high attenuation in the upload frequencies. Your attenuation at 300kHz is higher that the overall attenuation for all used frequencies. I've never seen that before.
A slow upload speed like you are currently seeing could definitely slow down your browsing speeds. When you try to download anything with those upload speeds, your computer probably can't respond fast enough with ACK packets. This would make downloading seem sluggish.
I think that you should post in AT&T Direct: »AT&T Direct and let the techs there look at your line stats. Good Luck.
Sorry this reply took such a long time but remodeling is taking a lot longer than originally planned (nearly almost done now). There was no difference when using my netgear router though.
When I switched to pro AT&T said I was just on the borderline of pro speeds and suggested we "give it a shot" to see if the line could take it. In the 2 years since then I have never had as much as a slowdown in speed or slight latency issue with the line, It has been rock steady until now. Seems that I'm not the only person experiencing troubles that lives in the Los Angeles County/Orange County area either so maybe something went wrong with a server or maybe there was an overload due to summer heat and more people on?
I will make a topic in the AT&T Direct forums as soon as the remodeling is done (won't be long now I hope) and also include a link to this thread. Thanks for all the help again.