Your line can only just achieve the full 80Mbps sync rate at 6dB SNR, probably due to crosstalk as you have suggested earlier, and there is absolutely nothing that you can do about that. Given the storms that have been going through the country in the last day or so I wouldn't be surprised if the error counts have been high enough for DLM to think that a resync is in order, and when it does the sync rate will change slightly each time, which is what you are seeing. Unless your speed drops significantly further, which could then indicate a fault, then I'd say that what you are seeing is just the way your line is now and you'll just have to live with it.
As a comparison below are my line stats. I've lost a good 10% of sync rate over the years, probably due to crosstalk too. I know it's just way VDSL is so I live with it. I suggest you think yourself very fortunate to have a line as good as you have.
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason: 1
Last initialization procedure status: 0
Max: Upstream rate = 6899 Kbps, Downstream rate = 43831 Kbps
Bearer: 0, Upstream rate = 6899 Kbps, Downstream rate = 43403 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 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): 2.9 5.3
Attn(dB): 26.6 0.0
Pwr(dBm): 11.9 6.5
VDSL2 framing
Bearer 0
MSGc: -6 26
B: 227 207
M: 1 1
T: 0 41
R: 10 8
S: 0.0000 0.9573
L: 11396 1805
D: 4 1
I: 238 108
N: 238 216
Q: 4 0
V: 0 0
RxQueue: 104 0
TxQueue: 26 0
G.INP Framing: 18 0
G.INP lookback: 26 0
RRC bits: 0 24
Bearer 1
MSGc: 90 -6
B: 0 0
M: 2 0
T: 2 0
R: 16 0
S: 10.6667 0.0000
L: 24 0
D: 1 0
I: 32 0
N: 32 0
Q: 0 0
V: 0 0
RxQueue: 0 0
TxQueue: 0 0
G.INP Framing: 0 0
G.INP lookback: 0 0
RRC bits: 0 0
Counters
Bearer 0
OHF: 0 886704
OHFErr: 3118 1846
RS: 1904366184 2629671
RSCorr: 401638869 7477
RSUnCorr: 0 0
Bearer 1
OHF: 229217369 0
OHFErr: 97 0
RS: 1375303845 0
RSCorr: 34213 0
RSUnCorr: 142 0
Retransmit Counters
rtx_tx: 459985677 0
rtx_c: 20264470 0
rtx_uc: 339508 0
G.INP Counters
LEFTRS: 8730 0
minEFTR: 43400 0
errFreeBits: 3727063533 0
Bearer 0
HEC: 0 0
OCD: 0 0
LCD: 0 0
Total Cells: 2187685029 0
Data Cells: 2548911881 0
Drop Cells: 0
Bit Errors: 0 0
Bearer 1
HEC: 0 0
OCD: 0 0
LCD: 0 0
Total Cells: 0 0
Data Cells: 0 0
Drop Cells: 0
Bit Errors: 0 0
ES: 714 2542
SES: 57 0
UAS: 91 54
AS: 3682895
Bearer 0
INP: 51.00 0.00
INPRein: 1.00 0.00
delay: 0 0
PER: 0.00 9.85
OR: 0.01 25.98
AgR: 43498.78 6925.53
Bearer 1
INP: 2.50 0.00
INPRein: 2.50 0.00
delay: 0 0
PER: 16.06 0.01
OR: 47.81 0.01
AgR: 47.81 0.01
Bitswap: 1701772/1701788 32282/32386
Total time = 91 days 17 hours 6 min 39 sec
FEC: 762036871 10409
CRC: 5058 3170
ES: 714 2542
SES: 57 0
UAS: 91 54
LOS: 1 0
LOF: 4 0
LOM: 25 0
Latest 15 minutes time = 6 min 39 sec
FEC: 54592 0
CRC: 0 0
ES: 0 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: 71624 2
CRC: 0 0
ES: 0 0
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0
Latest 1 day time = 17 hours 6 min 39 sec
FEC: 11592052 131
CRC: 26 22
ES: 17 21
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0
Previous 1 day time = 24 hours 0 sec
FEC: 12587814 516
CRC: 272 159
ES: 41 116
SES: 4 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 1 0
Since Link time = 42 days 15 hours 1 min 51 sec
FEC: 401638869 7477
CRC: 3118 1846
ES: 485 1472
SES: 31 0
UAS: 16 0
LOS: 0 0
LOF: 0 0
LOM: 15 0
NTR: mipsCntAtNtr=0 ncoCntAtNtr=0