Hi Ken,
The way I personally go about this is as follows ...
Reset your hardware to default, this is a case of a reset button or Reset instruction, (I prefer the hardware method, but that is just me)
Remove
any and all 3rd party helper and tuning applications from your system
Completely remove your PC's Network adaptors and re-install the drivers, (
obviously you need to download the latest drivers for your hardware, preferably from the manufacturers site, before your remove them 
)
At regular, (Morning, Mid day, Afternoon & evening), times over the day run a test at speedtest.net test on your main system and, if possible,
by running the same test on another device (a Laptop or phone - Speedtest.net is available on all major platforms, (either as an App or via a browser))
Make sure that Speedtest runs the test on the same source each time, (the standard run always goes to the server with the lowest latency, (ping)), you can select this manually if the test picks the Wrong One)
You can set up a free account on speedtest.net to store all your readings, (which I would recommend)
When you have a day or so's readings you can share these with your ISP, (EE), (
when you talk to them ask to speak to the Fibre Broadband Team directly).
Explain your problems and explain that you have done the above and offer to supply the data you have. I have found the actual team to be helpful and knowledgeable.
Hopefully they will instigate a full line test which will check connectivity from their location to yours, at all relevant points, and can get BT to check for line issues, between the Cabinet and your properties, recent changes in line contention factors Etc.
This may take a little while as there is involvement required from both EE and BT,
but definitely stick with it, the initial helplines at most ISP's are there to deal with general queries, and I suspect that yours needs a more in-depth approach.
This may seem a lot to do, but I have found, over many years, that Pre-emptive data collection on a base line system or systems means that the ISP is more likely to act rather than brush you off with excuses
