Im thinking its more likely BT Openreach moving my line around to get better stats for other customers....
Openreach use spare pairs when trying to get better speeds. They don't randomly swap users lines around.
It's not a Plusnet issue either and switching ISP will make no difference as it's the exact same line on a new ISP.
one night my internet went off at midnight dead on, came back on at 4am with no dial tone and a 10Mb loss in speed
Have you checked your cabinet recently to see if it has had any type of expansion work done?
If you are on a large Huawei 288 fibre cabinet the expansion is very subtle.
Openreach do upgrades through the night.
When they add High Density expansions to DSLAM's they usually do them after midnight.
They don't do routine work at that time of night.
It sounds very much like crosstalk to me.
A drop from 80Mb to 67Mb is within the typical range that can easily be lost to it.
I've seen lines drop from 80Mb to the mid 50's from crosstalk.
An HD expansion being added to your fibre cabinet increases the number of ports/users on the cabinet which increases the likelihood of more crosstalk.
I've attached a picture of an HD expansion on a large a Huawei cabinet. It's the couple inches added to the right hand side of the cabinet that are a slightly different shade.
