Confused re drops in broadband speeds

[#1]

I have been a broadband customer of plusnet for some 2 years.
For the most part after joining them I did not see any improvement in my speed from the historic 2mb/s that I got from my previous provider.
However over the period I have been a customer there has been repeated drops in speed down to nonsense levels of as low as 0.062mb/s.
I have usually made repeated attempts mainly reboots on router and monitor speeds till it self corrects or resorted to contacting the help team.
The last serious drop in speed some months back resulted in an Openreach visit which resolved the situation and as a precaution side stepped my extensive phone extension wiring and adopted a redundant business line.
Following this whether it be a coincidence my speeds have gone up at times by in excess of 100% to a max of 5.12 but normally in the region of 4.00.
This has not been without drops to the silly low speeds referred to earlier.
The most recent drop in speed has been taken up and resolved either by Plusnet or by the actions they asked me to undertake.
I am mystified as to what causes these drops in speed and am part convinced its more to do with BT than plusnet.
We are rural and all cabling back to the exchange is overhead.
If anybody has had similar experiences and has any suggestions I would be delighted to hear them.

[#2]

Hi there,
Sorry to hear about the problems you've mentioned. Really glad things seem to be fine at the moment though.
From our point of view we'd need to investigate this further to find more about what's causing this.
We're unable to do that if things are working fine in the short-term but I would advise "reviving" this thread if you start to get the same issue again.
While checking your account I decided to run some diagnostics which actually showed a mismatched speed profile, I've updated that manually to the correct one to avoid any issues related to that in particular.

[#3]

Hi Adam
Many thanks for the reply.
The next time it drops I will carry out the advise suggested by Plusnet on 17.11.14.
Namely
1. The disconnect of all equipment other than router/ethernet/computer.
2. Speedtest and diagnostics via BTW.
One of the things that baffles me is that during the recent episode Plusnet conducted their own tests over a 50 hour period stating no outages (this was not an issue for me as no outage was experienced at any time).
The crunch was that whilst my 20+ speedtests over the 7 day period produced down values between 0.063 and 0.196 mp/s the Plusnet test reveals a value of 2 mp/s.
Very hard for an aging punter to comprehend.
However, most pleasing to find this resource to help throw light on the subject. 
Mick Johnston