[I wish I could reply to the thread as a whole rather than to just an individual post]
It was a Draytek 2860n Plus. Firmware 3.7.8 seems to be problematic to some people in various way. I seem to have had it quite badly. I've learned my lesson now -- I'm sticking to using the OR modem until I'm happy with the firmware, so I can reboot as often as I like.
I do have some good news though! During testing, a suspected copper line fault was found, and it was decided to task an engineer to investigate (no access to the premises was required). This was yesterday.
Then, this morning, two engineers turned up out of the blue and spent two hours poking, prodding and testing (with a bit of time at the cabinet as well).
They were quite puzzled by it all - they could find nothing wrong with the line at all. But they seemed absolutely intent on resolving the issue no matter what, and before they left they had swapped my pairs, changed the faceplate (Mark III), provided a new OR modem (ECI rather than Hwawei) and most important of all they arranged for a DLM reset. I felt like I was being treated like Royalty. I was very, very impressed with them.
And now I'm back to 80/20 - yay! (max attainable was 107 according to one test they did, which I think is impressive at 250m - roll on vectoring!). Based on the 22ms base ping times, I am assuming I'm not interleaved but I don't know for sure.
However, I'm still seeing some insane latency on the BQM.
The part before the red was before they arrived. The part after the red is after they left.
I'm not worried about it right now - actual speed tests are perfect, VoiP is perfect, and real-life downloads are perfect. I suspect it may have something to do with the router, or possibly we may need to tweak the line a bit. At any rate I'm not going to worry about it for another 4 days or so in case it settles down. Does that seem like a sensible thing to do, or does that chart indicate something specific?
And thanks for everybody's input on this -- much appreciated.