My fibre 80/20 connection was once solid at 56Mbps but has been erratic since the end of March 2016. It improved with the introduction of Vectoring in early 2017 and worsened noticeably during and after Storm Brian at the end of October 2017. I reported the fault to Plusnet and recorded speed tests on the BTW website as directed. Since then the faults.plus.net website shows
"Thanks, you've given us all the information we need to investigate your problem" but I never heard back.
The connection was producing up to 20 million downstream FEC errors per day but more or less working ok despite the odd spontaneous reset. It was beginning to improve again, then a reset ten days ago resulted in a sync of 48998kbps. I've rebooted my router six times since then, on one occasion allowing three days uptime but on each occasion I get exactly the same speed down to the last kbps so presumably a cap has been applied. DSLstats shows an attainable speed of around 55Mbps.
Plusnet say that there is no cap on my profile but because the BT 'dslchecker' gives an arbitrary A range of 36.1Mbps to 24.9Mbps they don't see a problem and can't or won't submit a fault report to BTO.
I don't intend to reboot the router again, to see if DLM will intervene but is that realistic? It seems to be the only option short of jumping ship and hoping for better results elsewhere.