The longer the monitoring period the better.
That way, patterns of disturbance/interference and even one-off and/or sporadic incidents are recorded & can, on some occasions, be used as evidence of a line 'fault' that quick engineer or remote BT/ISP tests don't always capture.
The xx999 values you have reported do indeed suggest DLM intervention, so 'something' does appear to be impacting on your line's stability.
Increased crosstalk as more users have been connected over a period of time could be the cause though, rather than an actual line 'fault'.