Morning Bavon,
Best advise I can give is start with the standard Diags:
1) Test Socket
2) Filters/Faceplate
3) Modem
4) Possible Inteference
1 is self explanatory, 2 change the filters or buy an SSFP from ADSL Nation. 3 make sure the modem is ok or at least borrow one to see if you get a different sync speed. You can also try a factory reset on it. 4 check for interference, such as radiators, street lights and so forth.
If you try this and it doesnt help, ask your ISP to check the line and if needed report a fault to Openreach.
Leo