I get this problem as well on my D-Link router and did some research a while back. Hopefully this bookmark will provide some information and/or a possible fix if the cause it identifies turns out to be yours too:
http://www.kitz.co.uk/adsl/lowSNR.htm
This turns out to be what is causing my problem, because I have confirmed that when the line does drop out it is because the SNR Margin has got below a certain threshold, normally 12 as mentioned on that page. Solving the problem is another matter, though again that page has a number of solutions. At least it might help to know what the problem is though, hope this helps
