Some “hidden” Cisco commands for changing/adjusting the SNR from within the Cisco. I have used this many times as Cisco Routers are much more sensitive to change and issues than any other equipment.
Go into global configuration mode and type the following:
int atm 0
dsl noise-margin (a value between -3 and 3)
The dsl noise-margin forces the router to training at a higher noise margin (sacrificing speed for stability). Setting this to 3 for example should see you get a higher noise margin, slower speed (and depending on firmware) a higher attenuation.
See my other article about the 4.0.18 ADSL firmware as this fix combined with the latest firmware has resolved many of my Cisco 877 connection Nightmares (especially on a BT Internet connection)