Wildly fluctuating SNR after resync

Just wondered if anyone has seen this before?

My line is with Now Broadband (SKY), super fibre. Connected at 79.4 Mb/s for 3 months solid 6.2 SNR.

Yesterday afternoon, noticed a drop in connection. Router reconnected at 78.7 Mb/s but the SNR varies wildly. I'm using OpenWrt and the stats refresh every few seconds and on each refresh the SNR changes, I've a video if it helps but here are the readings from a few consecutive values each just a few seconds apart:
SNR 3.8 5.3 4.1 5.0 4.0 4.4 5.8 3.8 5.9 2.4 6.1 2.4 5.7 2.3 6.1 2.8 4.4

Max rate fluctuated inline with the above, other stats in case relevant
LATN 8.3 dB / 6.9 dB
SATN 8.3 dB / 6.9 dB
ACTATP -8.9 dB / 13.0 dB
LATENCY 0.18 ms / 0.0 ms

In an attempt to clear this unusual behaviour, I turned off the router overnight. This morning it reconnected at 70.522 and continues to show the same fluctuating behaviour albeit within a narrower range (4.5 to 6.9 SNR)

There are no changes to setup, all internal wiring is disconnected, modem in master socket, no known new sources of noise.

Still continuing, line seems strangely stable considering the swings in SNR

Just wondering, does

Aggregate Transmit Power(ACTATP): -9.2 dB / 12.1 dB

Seems normal? The negative downstream value, what does that mean?

Post deleted by RobertoS

In reply to a post by armistice:

Still continuing, line seems strangely stable considering the swings in SNR

Just wondering, does

Aggregate Transmit Power(ACTATP): -9.2 dB / 12.1 dB

Seems normal? The negative downstream value, what does that mean?

It means nothing ... power is never measured in pure decibels (dB) but relative to a fixed value. So, those could be dBu, dBm, dBw, dBM ...