There was a period when the modem and DSLAM remained in a synchronised state but the upper software layers were having severe problems. Possibly the Plusnet problems, as others have reported.
Doubt it. Admittedly I havent checked the PN forums of late, but as I mentioned in the other thread, the vast majority of those reporting problems seemed to have IPs in the same range. Being that since moving over to dedicated WBMC, the IP's are allocated to a more geographic location I thought it may be more localised and related to near to the MSIL's... when someone checked, they were indeed in a similar region down south somewhere.
Aside from that I dont think anything PN related could cause the upstream SNRm to vary like that. That 1dB spiking started on Sept 10th after a period of outage.
The modem
may require a full power cycle. You do sometimes see that happen across various modems, I think NS had one a few weeks back, and I've also seen someone else (using a Billion?) within the past month on this forum see the same thing and as I suggested it vanished after a power cycle.
As I said at that time, its something that I've observed way back to the days of just adsl and it seems to be more likely to occur if the line has been knocked out due to a power surge or after a power outage. Going back 10 yrs I had a Voyager 2100 router, which seemed to have a habit of doing this if my mains had been switched off and hadn't been shutdown gracefully first.
I too had one myself within the past couple of days. Something knocked me offline at 03:51. I dont know what caused the resync at that time, (possibly a storm - dont know was asleep).. but note that when my line automatically came back up it also showed that spiking. You can then see at about mid-day I had noticed it myself on DSLstats, so I performed a power cycle. It went as soon as I did a full power cycle. It seldom works with a simple resync and needs a full power down to clear.