Strange issue - down load speed limited when using router

[#1]

Hi All, I have a very strange issue with my fttc install. Since install on wed we are only getting around 18mb down but 6mb up via a router..... Let me explain further....
We had the install done complete with plusnet router. I swapped out the plusnet router for my own netgear dgnd3700 but am disappointed with the speed and bt wholesale speed test confirmed slow speed. I have swapped back to the plusnet router and still the same result.
Now for the strange bit, after chatting with support (thanks Bradley :)) I tried the PC direct connected to the modem with a PPPoE config and the speed test came back with 44mb down and 6mb up...... So I reconnected the plusnet router and re-ran the speed test and back to 18mb down????
So to recap, connecting direct to modem from PC will give me 44mb down but via either the plusnet or my own netgear router only 18mb down? Have also tried alternative etc cable from modem to router but same result...
Any suggestions?
Thanks, Chris

[#2]

That sort of thing is usually caused by a late Delta report putting your Current Line Speed back to ADSL levels, but if you are getting full whack connected directly to the modem, it points to a faulty router, or a dodgy ethernet cable between the two boxes - have you another cable you can try?
(ETA) I just noticed you have tried two different routers, so it's looking more like a faulty cable.

[#3]

Hi Jaggies, thanks for the reply.
Have swapped the eth cable between the modem and router just to be sure and same result. The thing that puzzles me is that I connected the PC direct to the modem using the original eth cable and got the 44mb down, so that sort of ruled out the modem and the cable.
The only thing I could think is that modem is somehow able to tell the difference between a PC and a router interface and is somehow limiting the down speed when it detects a router, but I have never heard of that before.....
Other things I can think of would be port speed on router, however we are getting 17-18mb down so it can't limited to 10mb and it is the same result on 2 different routers...
Chris

[#4]

Silly thought
You haven't got a WIFI card on your PC have you? The default connection is WIFI 

[#5]

Hi Peter, no it's the built in wifi and I am switching the wifi off to ensure it is running over the eth cable.
Cheers,
Chris

[#6]

Ok, quick update on this. I have re-run the PC direct to modem test tonight and am now only getting 20mb down so at least now I have consistency. I have tried a few different eth cables so have ruled that out.
Ticket logged with plusnet so will see where we go from here.
Chris

[#7]

You don't have any of the parental controls set in the router do you? There's a known issue with them limiting throughput massively.

[#8]

Hi Kevin, not on the netgear as I factory reset last night to see if that would clear the issue. I assume that plusnet don't put parental controls on their supplied routers by default?
Cheers,
Chris