Extreme drop in speed required router reboot

[#1]

We wake up to radio on our Sonos each morning.  However, this morning it didn't happen, as the download fibre speed had dropped to such an extent that the connection wasn't strong enough to start the radio.  On doing a speedtest (wirelessly) I found that the download speed was 0.64MBps, with the upload being 9.34.  The only way I could sort this out was to reboot the router, when the download speed want back to what I am expecting (35.52MBps).
We also have a Naim system which is connected via a PLA into a switch.  Again, before the reset I couldn't start off the radio (the Naim uses a different system than the Sonos, so I was trying to discount TuneIn from being the problem), but after the router reset, all was fine.
I can't tell you what gateway I was on before or after, but these massive drops in speed cannot be right....
I'm sure there isn't a problem with the line, since this was thoroughly tested by an engineer last week, and the fault at the cabinet was fixed.
Plusnet or anyone, could you shed any light on what's going on here?  Any issues reported in the cabinet?  Capacity issues at Plusnet?
Looking at the SamKnows stats, they show an extreme drop in line with what I have specified above.
Many thanks.

[#2]

Have to say I'm a bit disappointed that PlusNet have ignored this issue.  It's becoming even more of a problem - the only way I'm able to get around these extreme drops is by doing a soft or hard reboot, which of course could have knock-on effects with DLM.  For information, I've attached line stats for the past 7 days.
Could it be that the router itself is faulty?  I'm using a Netgear D6400 with firmware 1.0.22.
I'd appreciate some sort of response from PlusNet, or if anyone else has any ideas as to what might be happening, I'd appreciate hearing from you.
Many thanks!

[#3]

I'm sorry to hear about the issues you've had.
At the moment I can't test your line as there's some emergency maintenance going on at your exchange.
I'll update you when I can test it.

[#4]

Hi Matt
Interesting that they were doing work at the exchange - I couldn't find anything on the tracker.  Are you able to test today?
Many thanks
Mark

[#5]

Quote

Service found to be affected by existing network event.

Although I don't actually believe that result as the network incident it references shows as being cleared.
Let me speak to Matthew W as he may have an idea on this, he'll be in the office in the next hour or so.

[#6]

Could you please update me on this?
Many thanks.

[#7]

I have a Netgear R6250 that would get slow speeds in the morning, not as dramatic as yours, down from 34 to about 12 and I found the solution was altering the router's 'Connection Mode' setting to 'always on' rather than the default, which was either Dial on Demand or Manually Connect, can't remember to be honest.
Probably isn't the same issue, but worth considering maybe.

[#8]

Sorry for the delayed response.
Our tests haven't picked up on any issues and there isn't any congestion issues at your exchange.
Are you still seeing issues?