We had plusnet from when they first launched FTTC, line worked great at 36-40Mbps and 8-12Mbps upstream for first 2 years. Then in the summer (4 years ago) on the very hottest afternoons, it started dropping out.
Last year it was regularly on the hottest days, dropping the line to 25Mbps in the summer, despite the complaints and the intermittent nature, BT Openreach thought a 2 minute test at 8AM when the line always works was sufficient to dismiss the fault even though their equipment picked up a copper impairment in the last bit of line, they even tried to charge us when we agreed to try a router, even though we knew it wasn't our equipment. Their router was so bad, we couldn't even sustain the connection.
After trying multiple 6ft cables into the master socket 3 modems, and two routers - one replacing the modem, followed by an Asus router last year (which is absolutely amazing, blows all other brands out the water) we wrote a script to capture >100 vdsl2 properties a minute, charting 12 of them. The script showed clear issues, which a retired engineer suggested was uv insulation degradation (when we checked the times (3:30PM GMT 4:30ish BST) it correlated exactly with the sun crossing over so the house was no longer in between the line and the sun.
The router also showed massive attenuation changes sometimes long after the peak of the fault (and what might look like noise) hours later, the router during fault always reported 0 for REIN and SHINE - though it might be unsupported by that cabinet.
This year it started so badly it dropped to 17Mbps, then 8.8Mbps. We asked them to retrain the line like they did last year and they mysteriously said they now couldn't do that. I've also read that these massive slowdowns are happening to other customers, and may not only be the result of the fault.
Plusnet have said they will follow up with calls to explain what is going on, they didn't, multiple times, one even said the lack of action was extremely unusual. I also mentioned the USO March 2020 guarantee where BT are now obliged to provide a 10Mbps minimum service, despite repeated requests for the ticket logs for 2019 and 2020 and a letter so I could press BT into actually fixing the line, they didn't even put the requests in the notes, so I felt there was a clear conflict of interest too, they were obstructing my attempt to hold Openreach to their contractual obligations.
Even at 8.8Mbps we get a slight blip on hot days - which we're sure would have got worse, and we shouldn't have to wait for it to either fail, or for BT to actually upgrade to FTTP (which they have now delayed massively) The idea that a 2 minute line test sufficiently tests a modern complex broadband service for faults should be an embarrassment.
we spent those 4 years ruling out all other issues, we even put the (3rd) router on battery disconnected all lan and turned off almost all appliances during the fault - directly connected to the BT box with a shielded cable - it still happened, and I don't see neighbours calling BT out every other day so it must be line as noise doesn't randomly cause a huge attenuation change, we now sometimes see issues due to rain too.
You can see the charts from Aug last year at untamed.zone/adsl choose 21/04/2020 when the apparent noise event starts and ends (you can only see the attenuation at each resync) but you can clearly see there are issues. The routers have always been on top of a fan too and out of direct sunlight, so its not overheating)
BT Openreach didn't even bother turning up this year, claiming a remote test, which we never saw on the line stats, so we haven't been able to show them, so we figured Openreach have been lying about testing too, as of May this year, even though the lockdown is now lifted.
I took out a 2nd line with a provider who does it for £24mo not £60 like BT are charging. This should expose the fault as being on the line so I can try to claim back compensation/refunds from the ombudsman, and maximise the likelihood of fixing the problem. We can then copy the huge amount of data we have for the fault to our MP to ask for Openreach to be overhauled and separated from BT, I feel a political solution is needed to challenge Openreach
For the record, our line to exchange is about 715M according to ISP tests (though most online estimates predict 1400M) and we are well under 200M from the cabinet (120M by foot)
The underground cable and drop cable were installed less than 20 years ago. We're also paying £10 more than a new package from other ISP's