After over 12 months of frankly appalling service we found out recently that there is a system in the exchanges for 'capping' your broadband speed. It kicks in if the line suffers from unknown interference or something similar, on the basis that a consistent but slow service is better than one with breaks in it. (Or you might argue that it hides the fault if you're not switched on to the variations in speed.)
The cap will lower speeds from the alleged 8mb down to around 500k if it is triggered often enough in a short space of time.
We have pretty much rebuilt the system internally so as to rule out interference from DECT phones, PDQ machines etc., so now when we ring up and ask if the capping has activated again, we get an engineer.
To check your speed look at the BT Hub web page.
