With the information that you have given, it sounds that BT maybe right. They do not have full 8mb capability in all of their exchanges, and down-size services for short periods of time until the exchange has enough capability to serice its customers - (not my view, just what a BT engineer told me.) However, they can easily sustain a constant 2mb for subscribers to that level. I'm in the same boat as you!
However, in defence of BT, have you made sure your drivers are up-to-date?