joe -´'tis true - they will offer you all sorts of 'reasons' why you are having problems - BT line, computer distance from master socket, your machine, firewall etc etc. I had the same, but have limped through to the 12 months and am moving to 14.99 a month, 1 month contract and unlimited.
If you REALLY feel they have 'broken' their part of the contract, have you put it in writing? That often helps your case. I´m sure some legal wizz will help you here, but words like "failure to provide a service as advertised" and "product not of merchandisable quality" often help. Your contract (read the small print!) MAY require you to pay for the modem or return it.