Having joined plusnet last year my net has been a stable 17mb (don't laugh). Last week it dropped to 8mb and back up to 16mb after a reboot. Today I came home from work and my net had dropped to 8mb again . I rebooted and it's at 10mb now . I think I've a fault on my line
I have a similar problem but much more consistant.
When I first got fibre connection i was getting 17 to 20 Mbps, that was in 2015. Since then the speed has gradually declined, though upto October 2020 I was still getting up to 16Mbps. From October 2020 I have had a lot of connection problem, with connection dropping repeatedly and requiring reboot. Plus arranged visit by Open Reach engineer 3 times. The first 2 times they found fault and "fixed" it, only to recur again. The third visit resulted in supply of a new modem. I don't know how effective this will be.
The point, however, is that with the biginning of the connection problem, the speed also dropped to between 8 and 12 Mbps. Nothing I can do will raise the speed over 12.7 Mbps. I don't understand why the speed has gone down over the years and why it so low now.