Thanks to both for your input.
As per advice I shut the system down overnight.
Re-synch this morning has produced marginal improvement in US synch speed with associated average decrease in US SNRM, unfortunately, the SNRM oscillation remains the same.
I wonder if some major rearrangement of the physical location of the tie-pairs to D- & E-side cables has taken place within the PCP and your circuit has reacted badly to its pair's new environment?
As I monitor my line 24/7 and can positively identify that the re-synch event occured at 19:45 hrs. would think that it's unlikely, but not impossible that any local activity (PCP/DSLAM) was taking place at that time.
The PCP/DSLAM is located at the end of my road not within visual range of my residence.
either a broken UPBO or some severe US signal integrity issues, either way not good.
Pardon my ignorance, UPBO ?
Am I correct in assuming that it's unlikely that the issue could be local, ie. within my equipment/installation ?
I do have alternative hardware which I could substitute but really don't think this would be worthwhile ?
Is my ISP (Plusnet) likely to be interested in this issue especially as DS speed has not been effected, have not contacted them to date.
Have a very long standing administration issue with Plusnet but, although not recently required, have found their tech. support to be good.
Attached, SNRM & Bitloading graphs post latest re-synch. for reference.