join together the 3 parts
http://
community.bt.com/t5/
BT-Infinity/Snr-margins/td-p/492345

Just bought this:
http://www.ebay.co.uk/itm/220814370548?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Going to give it a go with the Draytek power supply connected to the filtered outlet. It seems a popular item for hi-fi systems and it may benefit the Draytek by giving it a clean power input.
Only cost £20 including delivery- so worth a try!

Update
The router has remained solid as a rock, since I moved it to it's own power socket. SNR 98 up, 65 down- all the time. Just got to wait now to see if DLM will allow my download sync to go back to where it was a week ago (79000mb/s). Strangely, the SNR does not change at night, as ADSL used to.
So- I have a Draytek 2850n running firmware 3.6.2_232201; IPV6 disabled; QoS on for up only; dedicated power socket- and it's proving to hold the connection throughout the day- result!

Nice report about its own power socket & the draytek...THANKS
if any changes keep inform us and if keeps staying stable
Not sure yet if will help much (so it looks like) but for sure almost all modems should plug in the electric socket alone as they doing sensitive work

the draytek to my eyes seems is very power full and can do the job easy but looks from time to time very unstable example when i reboot ... wifi sometimes coming weakest or less synchronisation or more much errors etc....if i reboot again is coming to nomral and rice versa each reboot
It makes think is the reason you have discovered causing issues as my draytek is plugged in an extension socket so is affecting its performance for sure

if your SNR stay stable after a week about, give a reboot manually(it should synchronise higher)