What would cause this leap in SNR?

[#1]

I periodically find my snr as displayed by routerstatslite rises or dips dramatically.
Like this:

I can't explain why this happens.  Anybody?
Thanks!
-- gyre --

[#2]

Possibly the DLM making a change due to intermittent noise on the line. Looks like a 3dB increase which is typical of the step-change that the DLM makes.

[#3]

If it was the exchange changing the default SNR margin from 6 to 9 then the router would have re-sync'd at the same time at a lower sync.
The OP doesn't mention whether this happened or not.
Once the exchange DLM has changed the margin higher it takes a LOT of time of stabiity before it will change it back down again.
The OP states this occurs up/down regularly.
I reckon this excludes the exchange DSLAM as doing this.
So I'd hazard a guess is some form of switching being picked up as interference on the line. Could be in the OP's house or indeed a nearby one.
So at 0816hrs on the graph something causing intererence switches OFF and hence the available noise margin increases as the interference ceases.
Try and see if there is any pattern to it, switching at constant times in the day for example - faulty thermostats on heating/hot water systems have been known to cause this sort of thing.

[#4]

The default SNR margin doesn't change. It is 6.
If I reboot the router, it will re negotiate at 6, and my sync speed will increase from 3200 to 3700 kbps.
I'll start watching out for a pattern.
Thanks.
-- gyre --