Sorry for being daft but what do you mean by sync speeds? If its the data rates they seem to be steady.
You're not being daft just because you didn't know what I meant. Sync speed is the rate at which your modem synchronises to the exchange or cab at. Some routers will display it as line rate and its totally different from throughput speed. Throughput speed is what the likes of the Ookla speedtest will give you.
4am is usually the DLM, but without having the sync speed from your router to compare its difficult to say. Unfortunately the TD-W9980 gives little in the way of line stat information.
ejs has written a small java application which can get a few more details. Information in
this thread.
I'm taking a wild guess that DLM applied some form of action (such as interleaving) on the 8th, but if your line has been stable for 3 days, then it will reverse that action and thats why speeds are slightly higher again.