Only way to know is to get a fault reported and things investigated.
If you have a single socket and connection speeds are no different using the test socket and its not failing modem or failing microfilters then not much more you can do.
One final thing to try is turning off everything you can in the house, i.e. true off and not just on standby and this includes power to boiler i.e. so only thing left using power is the ADSL2+ modem and see if things are more stable.