Getting half speed and sync dropping

[#1]

Matthew thanks for the suggestion, I might have found the culprit.
I completed a PPPOE connection directly to the BT router and that showed a significant and consistent increase to current sync speed >60Mbps. (tested on TBB, Speedtest.net, BTW)
I plugged my Pusnet Router (TG582n FTTC) back in and Reset it using the small button at the back, however the speeds still dropped back to <40Mbps. (test again on all 3 sites)
I done the test again and speeds increased with PPPOE and decreased with router, everything in between stayed the same. Back to Plusnet router and speeds down again.
Is pointing to a faulty router now?  is it possible to have this replaced?  also I would like to have my line speed back set back to 77Mbps please as it dropped to 66.8Mbps this week after "investigations".
Thank you for your help.

[#2]

Unfortunately the routers almost 2 years old and as its out of its 12 month warranty I can't send a replacement free of charge.
If you want to get another router from ourselves you'd have to speak with our Customer Service team to get this done.
Alternatively there's nothing stopping you from using a 3rd party router.

[#3]

Well if Plusnet don't cover it, I don't mind forking out for a new router.
do you think its a router issue though?
Not a firmware issue, not G.INP related stuff, or other router related thingy ma jing.
And what about the increase back to 77Mbps?

[#4]

If the router wasn't at fault then your throughput would have been poor when you were connected directly to the Openreach modem.
With regards to the increase back to 77mbps unfortunately there isn't much we can do about it. Have you tried rebooting the Openreach modem to see if that gives you a increase?

[#5]

So I got another Plusnet FTTC router and still getting under <40Mbps with Desktop and Laptop (direct ethernet)  
PPPoE bypass still gets me roughly sync speed at 62Mbps+
Why would adding the plusnet router to the mix drop me to <40Mbps it looks like a cap, because its just under 40Mbps every single test, does not fluctuate wildly at all.
So is the cause Plusnet, BT or still possibly my equipment/router config?
Any other tests, I'm happy to jump through all hoops to get this sorted.

[#6]

Matthew - can you tell me if G.INP is enabled on my line?
I believe i'm connected to a Huawei cabinet.

[#7]

G.INP affects the connection between the modem and the exchange so it wouldn't be the reason for the difference between the router and PPPoE. Have you investigated MTU settings which might be a reason for that difference?
G.INP could account for the 77 to 62 drop. What make of modem do you have?

[#8]

The line does have G.INP enabled however as Jelv has pointed out that won't be causing the issue as if it was you'd see poor speeds over the PPPoE connection to the modem.

[#9]

Quote from: jelv

G.INP affects the connection between the modem and the exchange so it wouldn't be the reason for the difference between the router and PPPoE. Have you investigated MTU settings which might be a reason for that difference? G.INP could account for the 77 to 62 drop. What make of modem do you have?

I have the TG582n FTTC -> ECI modem (not Huawei)  -> connected to a Huawei cab with G.INP enabled.
My MTU on the TG582 is 1492 for WAN and 1500 for LAN.
Windows 8.1 machine MTU was 1500, I changed it to 1492 but same speed @ 40Mbps on the dot.
I'm getting the Huawei to rule a faulty modem too.  Sigh!

[#10]

Got a Huawei and sync speed returned to normal.
Matthew - can you change my plusnet profile to match please?
thanks.