Having played with my 2850n and various firmwares to try and find the best configuration (least errors/highest sync) I thought I had found the answer in an annex B version of 3.6.3.
However, despite trying to leave the router connected for as long as possible to try and get my download sync back up to what it used to be- it rebooted at 4am and is now down to 59000 ish instead of it's original 79000ish three weeks ago. Does anyone know how to get my sync speed back up? Are BT sending some commands during the night which is knobbling my router?
Conspiracy theories......

Even i have adsl i have notice the last 3-4 days i have 1 disconnection at night and tonight had 2 all ready
Don't believing is our machines...so long did not make me problems unless is the new firmware 3.6.3 but i don,t think cos is using the same old vdsl version 1412f0 as 3.6.2
EDIT
Forgot to say...i have also changed to an other modem and still getting disconnections at night
If you getting disconnections will not help the speed go up (mentioned tip ,at your other post) until it stops

Interesting update.
Got round to installing my router on my bargain EBay APC UPS. Guess what? Three days solid connection with no reboots. Maybe overnight power glitches/outages were causing my reboots? Trying to keep the connection for as long as possible to try and increase my speeds. Fingers crossed!