I have had a BT Home Hub for some years and generally it has performed very well. I get a fairly consistent speed of about 16Mbs which is fine. However, every so often (every couple of weeks or so) the speed drops to about 5Mbs. Pressing the re-set button restores full speed immediately. As it is so easy to deal with it is not really a serious problem, but I was just wondering whether it was a common occurence or not.
It is possible there is an intermittent fault on your line. When you reset the hub, it goes at max speed then, if the kit in the exchange senses a problem, it backs off the speed until a stable state is reached.