If you've just signed a new contract this is how I'd go about it.
Check out other ISP's and pick one.
Check your BB speed, I use thinkbroadband; click here you can keep records using their link url. This is my latest check result on 2 meg connection; click here
Make a note of the dropout times and slow speed.
After a couple of days I'd ring them up, armed with the info, and tell them that you wish to cancel the contract as they aren't able to supply a decent service. Ask for the cancellation to be confirmed in a written letter and also ask for the mac code for migrating to another ISP.
You will have to send the router back.
As I said, this is just how I'd go about it.