Fibre goes slow, dsl Flashes then Internet Fails Altogether

[#1]

I've a FTC fibre network that has been great up to about 3 weeks ago.
Suddenly the speed dropped from ~23M to about 8.7M, then internet disappeared altogether.  The phone was VERY crackly and noisy!
I found the internet had dropped, and on investigation found the Internet light on the PlusNet router wasn't on, and the DSL light on the BT router was flashing. After a while the BT DSL became steady, but still no Internet. Powered the PlusNet router down, then up and after the usual wait all was back to normal (and ~23M)
This situation has been repeated a few times since but WITHOUT the phone line apparently playing up, each time the speed drops to about 8 - 9 M, then the BT router DSL light blinks and/or goes off. After a while the BT DSL light comes back on but there's no connection to the internet UNTIL the PlusNet is powered down & back up (is there a way of kick starting the internet connection via the plusnet router interface?)
It's becoming difficult to rely on the connection, can anyone suggest what checks I can do or what can be done at the PlusNet end
Thanks
update; 28/Nov/2015@19:20;  3 dropouts in last 20 minutes, phone line is crackling again
[Moderator's note by Dick(Strat) All capstext in title edited as per forum rule

[#2]

You should report a PSTN voice telephony fault to who ever you pay line rental to, as if you can here audible noise  it usually can be evidence of a HR type fault, but to exclude the filter  remove the face plate on the master socket and plug a corded phone if possible into the test jack and do the quiet line test again, if you can still hear noises then report a fault

[#3]

I can see we've raised a phone fault to our suppliers.
We'll get this fixed as soon as we can.