I had a similar problem a couple of months ago.
I got a speedy reply to my email asking for help,the important part of the reply was :-
"The line is currently achieving a 1db SNR on a 6db profile, which i can see is causing the attenuation to be quite high on this line.
Certain routers are unable to cope with a 1db SNR and will drop the connection, please can you advise what router you are currently using?
I can increase your SNR profile to 9db, which is much more suitable for a line of this length (3092m), but doing so will decrease speeds to increase stability.
Please can you advise how you would like to progress this?"
I changed back to an old ZyEXL Prestige 660Series router and have had no problems since.
As I have posted before,speeds and service from COMS are excellent and my phone bills are even less than I had with the low ADSL24 prices.
I am the only one satisfied with COMS?