download/upload speeds have dropped - can i change my own SN targets?

[#1]

hey all,
for many months ive had a cracking (for me) 3.7M download / 0.7 upload with SN of 5 / 9db up down.  this has been great and in line with my synch which is:
Bandwidth (Up/Down) [kbps/kbps]: 871 / 4,544
However,  recently (last 4 weeks) my SNs have jumped up to 6.5 / 11 and as a result my download has been slashed to 2.2M so i can no longer stream HD content from xbox live video marketplace
with the Thomson gateway router can i manually go in and change the SN targets, otherwise can a plusnet rep do it for me?  my connection was rock solid at the lower SNs so cant understand why its changed.
many thanks.

[#2]

Certainly download SNRs can be changed by the user - I do so on my connection which uses a Belkin router. Don't know about the Thompson, will leave that for others to answer.

[#3]

The DMT tool can be use to lower SNR I believe.