Xbox Live connection dropping

[#1]

I've currently got a fairly slow internet connection (about 2mb, the best we can get where we live) and have been using it without issues for 2 months or so. However recently I'm having issues when connecting to Xbox Live via wired or wireless connection. Previously I've been able to play games online at the same time as my fiance without a problem, but lately we can't do this. If I try and play a game while she is even browsing the net on her iPad my voice chat drops or gets jumpy and I get disconnected from my friends repeatedly.
According to this (admittedly old) article: http://community.plus.net/blog/2012/12/21/its-unlimited-why-is-it-still-traffic-managed/ gaming services are supposed to be given highest priority. This is clearly not the case, her wireless browsing or YouTube videos are playing without issue while my game is getting disconnected and my voice chat is cutting out endlessly.
Any idea what might be causing this? We've tried a router reset but that hasn't worked, and according to speedtest.net our connection speed and ping have remained the same as always! It's only started getting bad over the last week or so.

[#2]

Hi Lloyd71,
I've had a look into this for you and we can't see any reason for the issues you're experiencing.
Would you be able to run Wireshark and capture the data from your Xbox live connection when the issue is occurring?
Please find a guide to running Wireshark here: http://community.plus.net/library/faults/using-wireshark-for-data-capture/
You would need to set up port mirroring on your router so that the data that's coming through the port where your xbox is connected will reflect on Wireshark on your PC/laptop.
:eth switch mirror capture port 3
:eth switch mirror ingress port 4 state enabled
:eth switch mirror egress port 4 state enabled
This is to capture on port 3 everything on port 4
i.e. Xbox on port 4, Wireshark on port 3
You would need to start to run Wireshark before you start the game and then stop capturing once the game has started. Please then save the file and then zip it and then submit this on a ticket for us to look into.

[#3]

Although what Linn has said is actually a good idea generally, we're now of the opinion that it's not really worthwhile on XBOX or XBOX one traffic. We've got one here in the office, so can check the traffic ourselves and are pretty confident that the sig is still picking this up.
Can you go through the faults checker at the following link so we can get your line tested: https://faults.plus.net