If it's a BT Wholesale (BTW) FTTC connection, you could ask for a Lower Threshold Breach (LTB) fault to be raised:
http://www.ispreview.co.uk/index.php/2012/12/when-sl...
"The 25% Rule
A lesser known fact is that of the 25% rule, which usually applies to services that initially sync up at 15Mbps Downstream or above. According to BTWholesale, �if the line rate drops by more than 25% over a 14 day continuous period then a fault can be reported�. In practice it helps to have a vigilant ISP on your side for this one as they�re best placed to provide BT with the necessary data."
EDIT: 85Mbps minus 25% is 64Mbps... at 67Mbps you're just over the threshold. But if diagnostics show 40% packet loss provable to the FTTC line this should still be raised to BT Wholsale as a line fault.
EDIT: Assuming this is a BT Wholesale connection there's a whole section on packet loss at:
https://www.btwholesale.com/assets/documents/Orders_...
From the decrease in sync speed and increase in packet loss it sounds like a copper line issue and not backhaul ('hot VP') related. A line test *should* pick up any physical line faults. Caveat: it's impossible to be conclusive about this without access to ISP diagnostic results from line tests, logs etc
Edited by deleted (Tue 07-Mar-17 11:21:18)