Curious. How long has it been spiking like that for? What modem are you using?
On occasion we see spiking like that on some VDSL lines after a power outage. It doesn't seem to be related to modem model as we have seen it on the HG612, Billion and Zyxel modems.
Over the past couple of months I've seen several patches of similar on my own line.
It usually starts at about 4-5am when the router will resync, then when it comes back up it starts the spiking... although mine is not as bad as yours (1-2db).
Usually the advice is to reboot the router and it comes back up fine.
However that didnt work for me a couple of weeks ago until I left it switched off for a couple of hours. -
see here.
I had it happen again last night - a reboot has stabilised it some, but its still jittering a tiny bit. I'm currently waiting until tomorrow to check on something on my phone bill, because I think that the time it started last night may have been when I phoned a friend, but that certainly wouldn't account for the others which all started at 4-5am.