I have been working on some xDSL sync issues and and here are some of my findings to boost the xDSL Sync rate and have a stable line.
First of all, make sure which standard your ISP’s DSLAM running on. This is important because some standards have limitations which will create a bottleneck when it comes to the hardware you are using. First rule of thumb for people who are not sure is to use the ISP’s provided hardware.
For example ITU G.992.5 (ADSL2+) will have an upload limitation of 1.3 Mbps, you might have a line which is capable of handling more than 1.3 Mbps but the hardware you use will cause a limitation. This is why you need to make sure which standard the ISP is running the DSLAM on. If the ISP happen to run ITU G.992.5 Annex M (ADSL2+ M) you are likely to get an upload speed of more than 1.3 Mbps, thus having a router which is capable of supporting Annex M will be beneficial.
Run your ADSL line from the Master Socket. This way, you are likely to eradicate any noise on the line which will affect the DSL connection.
It is highly advisable to remove the ringer cable on the phone line. This cable causes a lot of noise on the line and no longer required because the DSL Micro-Filter takes care of the ringer.
Only connect the wires on (2) and (5) on the BT NTE5 box and disconnect all the other wires. This way, there will be no static looping back via the ringer cable.
I would recommend going through all three Options above to make sure those are ticked and If you require any further assistance, I recommend you to do a Google Search and there are plenty of information out there which explains in more detail.