Have had same problems with BT and have complained several times each time after a few days they phone and say they will reset line speed and increase it gradually in stages by about 500kb a time due to automated sensing equipment and not to switch off the hub which I don't anyway. After a so called upgrade to the new BT n tech hub v3 my line speed dropped to below 1MB. They increased it to 6.3MB after complaining which lasted a few days and has dropped back to below 4.6MB again. I am convinced that BT throttle back line speed at the local exchange. Like you totally exasperated and just laugh when they talk about 20MB BT Infinity. Time they got their 8MB working! The only thing to do is to continue complaining or switch ISP.
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