We have had the same problem with BT Infinity. With some detective work I found that our cabinet is in Kings Cross about a mile away. Anyone passing through that area can see the huge developments and I guess we are having to share with many more. Our speed in the evening has become very poor, about 1/10 of what the glorious adverts promise. I challenge BT on this several times, without any improvement. Like many utilties there is small print in the terms and conditions which say basically they do not promise anything. Service level agreements (TLAs) to domestic customers basically do not exist. This need to be challenged in the courts, something I am planning to take up with a group of neighbours, some of whom have friends in high places. Stop overloading the cabinets, put more cabinets in, stop blaiming the customers for faulty this and that, stop blaiming it all on Open Reach, wake up to your responsibilities, stop paying big bonuses to the directors, improve your CRM, give us proper TLAs etc etc. Its 2017.
I'll be happy to take a look at this connection problem for you. I'll need a few details first. Can you post your helpdesk details? You'll find them in Troubleshooting/Helpdesk on the web interface at http://192.168.1.254 or http://bthomehub.home.
Can you try running a speed test, including the further diagnostics, and plst the results? (remember to remove any personal information).