Sudden drop in speed

[#1]

Sync OK then, but the re-boot was un-necessary. The profile will go up when you have been synced at a higher speed for a period of time. For small increases in sync speed this could be upto five days, for very large increases it can be hours. Typically about 3 days for average increases. Re-booting could lengthen the time as the exchange may think you are not sufficiently stable. It was just coincidence if your profile went up just after your re-boot.
Let's hope things now remain stable.
Happy new year.
Chris.

[#2]

Reconnected the extension to the master socket today (there's only one, a cordless phone) and the sync rate dropped from approx 7300 to 6100, so I'm guessing the extension could be adding noise to the line?

[#3]

Hi Martin, yep that's highly probable. On your stats what was the Downstream Noise margin (SNRM) just after sync when you reconnected the extension?
Is the bell wire (an orange with white marker, or pure orange depending on the age of the wiring) connected to terminal 3 on the back of the master socket front plate?
But particularly replace the cordless phone with a corded one to see if there is any difference, if there is (and the bell wire isn't connected) you may need to consider double filtering the cordless phone, which sometimes cures such problems, assuming there is no fault on the wiring. Best to do these checks and resync in daylight hours when noise levels should be lower. (Note when you change the phone, if the line doesn't drop and resync, check the SNRM immediately before and immediately after the swap. Providing it increases by around 1dB or more, you will then need to re-boot to see an improvement in sync speed.)