Ive just gone through all this, even got a new Blue modem, and some very good customer care indeed.
BUT, it seems theres a lot of this about, mine went slower than dial up over Christmas, the engineer said he had seen a lot of this recently, and doubted that a new modem would fix it totally, but at least they made the effort.
Its really variable atm, and its nowhere near the solid reliable 3meg i have enjoyed for the last year, i hope it gets sorted, i dont really want to consider another provider.