SNR slowly dropping

[#1]

Anyone got any ideas on why for months now my SNR is slowly dropping?
I had a line reset carried out some 6 weeks or so ago and my SNR has once again been slowly dropping. My last re-sync was almost 16 days ago when the up and down SNR was approx. 6.5dB, over that three week period it has now dropped to some 5dB down and 5.7dB up, I'm also seeing erratic spikes and drops in the SNR at irregular intervals which I monitor through DSLStats and my ZyXel SBG3300-N. I know from experience his trend will continue until I get reset to 6dB and the sync rate will drop to lower levels.
Appreciate the SNR fluctuates over a 24 hour period but the overall trend is constantly downwards and has in the past caused my sync rate to keep dropping. The telnet data e.g. attenuation etc. does not change so not that and the quiet line test seems clear.
I should also add that the SBG3300-N is rock solid and holding the connection without too many errors but even that will drop eventually if this continues, any help would be much appreciated.

[#2]

If it was on ADSL I would have suggested that the bit swapping was losing the higher tones over night and not repopulating them properly
How that relates to fibre I have no idea

[#3]

Appreciate the response but the tones are recorded when I start DSLStats and there seems to be no difference (or at least little difference) on a daily basis. As you say though not sure if this relates to FTTC anyway.

[#4]

Crosstalk?
Also I cant see the type of spikes to be able to judge so Im just guessing.      BUT..  FeXT is the bane of VDSL.  Ive lost circa 30Mb of attainable speed since I got VDSL purely down to crosstalk.  Everytime a new user gets added to the cab theres a chance that your SNRm will slowly deteriorate. 
I had a bad crosstalk disturber join the cab just before xmas.  I instantly lost 1.5dB of SNRm and about 5Mbps of speed.  Its quite funny because I can tell each and everytime the other guy turns his router off, or it performs a resync as my SNRm spikes back up to what it used to be.      The average loss now through crosstalk amongst users on my forum is running at about 15-20Mb.  It just slowly chips away at your SNRm  😕

[#5]

Quote from: Oldjim

If it was on ADSL I would have suggested that the bit swapping was losing the higher tones over night and not repopulating them properly How that relates to fibre I have no idea

The theory is exactly the same OJ.  Just lots more tones available for use.

[#6]

Thanks for the response.
The spikes I see only happen once or twice a week so not sure if that is crosstalk as the rest of the time the SNR just slowly changes over the 24 hour period each day, dropping late afternoon by around 0.5dB and then moving around by around 0.1 - 0.2 dB but over weeks the SNR never recovers to the same (or similar) upper level. The trend is maybe a drop (on average) of around 0.2dB per week.
At present my downstream SNR at best is around 5.8 or 5.9dB early morning, this was 6.5dB just 3 weeks ago.
Telnet data at present is as follows, I'm approx. 300m from the cabinet and Openreach tell me the line is pretty straight to my house:
xdslctl info --stats
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    0
Last initialization procedure status:  0
Max:    Upstream rate = 8598 Kbps, Downstream rate = 55430 Kbps
Bearer: 0, Upstream rate = 8898 Kbps, Downstream rate = 47671 Kbps
Link Power State:      L0
Mode:                  VDSL2 Annex B
VDSL2 Profile:          Profile 17a
TPS-TC:                PTM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        5.2            6.0
Attn(dB):        21.7            0.0
Pwr(dBm):        13.0            6.3
                        VDSL2 framing
                        Bearer 0
MSGc:          18              29
B:              51              238
M:              1              1
T:              64              13
R:              12              16
S:              0.0347          0.8543
L:              14752          2388
😧              933            1
I:              64              255
N:              64              255
                        Counters
                        Bearer 0
OHF:            687969399              549494
OHFErr:        5275            158
RS:            26393394                2655628
RSCorr:        60252736                8031
RSUnCorr:      238564          0
                        Bearer 0
HEC:            52871          0
OCD:            6              0
LCD:            6              0
Total Cells:    3043845497              0
Data Cells:    1286760820              0
Drop Cells:    0
Bit Errors:    0              0
ES:            1019            144
SES:            1              0
UAS:            26              1025
AS:            1534353
                        Bearer 0
INP:            3.00            0.00
INPRein:        0.00            0.00
delay:          8              0
PER:            2.22            13.93
OR:            86.10          20.09
AgR:            47757.44        8917.82
Bitswap:        738492/738493          372/379
Total time = 17 days 18 hours 12 min 59 sec
FEC:            60252736                8031
CRC:            5275            158
ES:            1019            144
SES:            1              0
UAS:            26              1025
LOS:            0              0
LOF:            0              0
LOM:            0              0
Latest 15 minutes time = 12 min 59 sec
FEC:            48931          0
CRC:            31              0
ES:            2              0
SES:            0              0
UAS:            0              0
LOS:            0              0
LOF:            0              0
LOM:            0              0
Previous 15 minutes time = 15 min 0 sec
FEC:            28391          0
CRC:            5              0
ES:            1              0
SES:            0              0
UAS:            0              0
LOS:            0              0
LOF:            0              0
LOM:            0              0
Latest 1 day time = 18 hours 12 min 59 sec
FEC:            1087708        5
CRC:            100            3
ES:            20              3
SES:            0              0
UAS:            0              0
LOS:            0              0
LOF:            0              0
LOM:            0              0
Previous 1 day time = 24 hours 0 sec
FEC:            1399087        7
CRC:            190            13
ES:            43              13
SES:            0              0
UAS:            0              0
LOS:            0              0
LOF:            0              0
LOM:            0              0
Since Link time = 17 days 18 hours 12 min 33 sec
FEC:            60252736                8031
CRC:            5275            158
ES:            1019            144
SES:            1              0
UAS:            0              999
LOS:            0              0
LOF:            0              0
LOM:            0              0
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