Noticed it started going wonky last Tues/Weds. I'm guessing that either someone bought something new that's using a f-ton of data continuously, and/or it's someone new on 68/SDO that's doing that. Eventually that person's going to get throttled. If it takes 20 days to get throttled, we'll know it was the latter.
How I know it was last Tues/Weds: I had two 3GB macOS updates, an iOS update, a 3.5GB Xcode update, a whole bunch of apps, a mac Office update, and a Win10 update all in one day - and all of it was much slower and sinusoidal than normal - even at 3am.