Asus DSL-N55U vs Billion 7800 single thread speeds

Been playing with the above two routers the last few days and the ASUS maximizes the line on single thread downloads. Even the TBB speedtest shows single thread downloads as nice and smooth. But even with the stability tweak I cannot sync stably above 16.5 mb/s and that is at 1.5db SNR, but unfortunately after 10 minutes the SNR drops to 0.5db giving me less margin to play with.

I have been using the Billion 7800 in bridge mode and it can hang on to 18 mb/s at 1.5db for weeks at a time if not more, but single threaded downloads sometimes slowdown for a second or so and the TBB speedtest shows the single thread download is variable and looks congested. TBB x6 is fine.

Has anyone got any ideas why the Billion is not so good on single download threads and the Asus is not so good at getting a stable high sync and what is the best configuration to adopt, ASUS on its own losing 2 meg, or asus with bridged billion gaining 2 meg but having to use a download manage with multiple threads to get the speed?

A helpful soul at the Asus user forum has suggested turning off bitswapping in the advanced dsl settings as this may give better stability and not drop the SNR so quick which I am now trying. Had about 12 hours DSL uptime today with the 5db tweak but lost carrier about midnight. According to routerstats lite after 10 minutes of DSL uptime the SNR seems much more stable at 1.3-1.5db with bitswapping off.

What is bitswapping?