I also have the Billion 7800N, not sure if you are aware, but you can tweek the settings to give you more download speed. I have upped mine through tweeking by around 1500kbps. You do it by adjusting the SN ratio. It is all to do with the chipset it has. I went from 6500kbps to 8000kbps by mucking around with it and no dropouts. Apparently the chipset in that router is a hoot at handling unstable lines.
Here is the link to the page that discusses it:
http://www.increasebroadbandspeed.co.uk/billion-7800n