An ADSL line and FTTC line have a 'profile', this can sometimes be adjusted or reset.
There is manual control over TT ADSL line profiles, find out all about it
There is some control over line profiles for BT ADSL lines, mainly an SNR reset
There is limited support on managing the profiles on FTTC lines, but find out more anyway...
Adding Interleaving may improve the line quality, removing it will reduce the latency, find out more here.