Well, you can only tweak the Target
SNR Margin, i.e. the one it initially connects with, see http://www.kitz.co.uk/routers/dg834GT_targetsnr.htm
If it then falls by as much as 9 dB, you have a noise issue, and all you can do is raise the initial Target SNR with consequent loss of speed.
There is no way to fix the SNR Margin at 9 dB, as it responds to noise over time.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 17 Meg Untweaked 19 Meg Tweaked WBC