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Turn your modem off for twenty minutes and then turn it on and try another speedtest. If your IP profile has not improved you probably have a line fault that needs an engineer visit.
If rebooting the modem does not help, then it could be a modem fault or a line fault. If that is the case please contact the mods on this forum who will help you, please use this link to contact them, thank you: http://bt.custhelp.com/app/contact_email/c/4951
Your line is identical to mine, same download + upload. Exact same IP Profile, I use to get 27Mbps stable but there is some damaged aerial cable that is damaged in my are and are waiting for Openreach to replace. Where abouts do you live?
Have you tried contacting the moderators?
my guess is its crosstalk.
crosstalk is brutal on vdsl2.
You are correct that when you upgraded to 80/20 service then DLM would have been reset meaning your 17mbit sync was probably the maximum.
eg. when I first got connected I had an attainable sync of 110/36. Very good.
Then about 2 weeks after I got activated this dropped to 90/36 (still 80/20 sync).
Then a week after that it again dropped to 73/24 (causing my actual sync to drop to 71/20).
None of the drops have been caused by DLM, simply the QLN on my line increased which is probably crosstalk from other lines. I was the first activated in my cabinet so I started off with no crosstalk, for reference BT estimated me at 65.9 down and 20 up.