Go into global configuration mode and type the following:
int atm 0
dsl noise-margin (a value between -3 and 3)
Most likely you will need to increase the SNR for stability. You should check the SNR via the command show dsl interface atm0/0/0 or something like show controllers VDSL 0 (or what ever atm you need this doing on). To increase the SNR try settings dsl noise-margin 1 and then check the SNR value again. You can increment it by .5, so maybe 1, then 1.5 etc.. It will resync, and you should see the SNR increase. You can gradually increase until you have a stable connection.
Note that you will get a warning doing this – WARNING: Unsupported Command. May cause violation to ADSL standards.