Connection speed dropped since the weekend.

[#1]

I had a zippy connection up until the weekend then I noticed a pretty dramatic slow down.
Since Saturday, iPlayer is a no-go. It stops every couple of minutes then throws up an 'insufficient bandwith' error.
My router said I had a DownStream Connection Speed of 19442 kbps and an UpStream Connection Speed of 444 kbps.
I ran the BT speedtest and it said my downstream speed was actually 1026kbs of a possible 21000kbs.
I figured it might be a temporary speed issue, so I let it go.
Today it's even worse, I can't watch youtube and even some facebook apps hang.
My router still gives me the DownStream Connection Speed of 19442 kbps and UpStream Connection Speed of 444 kbps, but the BT speedtest gives me a downstream speed of 513kbs of a possible 21000kbs.
The weird thing is that my upstream speed is (relatively) high at 360kbs of a possible 444kbs.
I don't think this could be down to either traffic shaping or general internet traffic, I had the same drudging speeds at 1AM last night.
I haven't changed anything at my end, all connections and setup is the same.
Help! What do I do?
I'm only one step away from checking the wires for kinks, like you would with a hosepipe 
John

[#2]

Hi feroluce,
From what I can see, this may be a fault. I can see you raised a ticket via the fault checker yesterday so lets see what the faults team come back with. Please reply to this post if no one comes back to you before the weekend and I'll muscle in if I need to.
Adam

[#3]

Hi Adam,
I haven't heard anything yet and my connection has gone even more silly.
Wikipedia pages take up to 15 seconds to load and I can't run the BT speedtest anymore, it just hangs.
My router is still showing a speed of 19211 but I'm getting 380, with an upload speed of 387.
Maybe it's actually an ISDN line
I'm going to try a shutdown and unplug tonight to see if being off the network for a time resets things.
I'll disconnect the phone too, just in case there's an issue from that.
I was holding off on doing this in case it interfered with diagnostics from the server end, but they don't seem to be happening anyway.

[#4]

HI feroluce,
I've just nudged one of our faults guys who'll test it shortly and give you a call.
Jojo

[#5]

I've had all the tests and it looks like my router is to blame.
I'm currently waiting for a new router to be sent out but the plot thickens.
I'm still getting a regular download speed of around 500K except......
Every night between 3am and 7am I get full speed.
Could this really be a router problem?
Surely if my router was at fault, the problem would be consistent.
I use the same router between 3am and 7am that I use all the other times when it's stagnant.
My neighbour told me that a BT engineer was working on the big network box on our street the day my connection went awry.
Could he have changed something?

[#6]

HI feroluce,
Add this information to your ticket and I'll get someone to pick it up.
Jojo

[#7]

P.S. Let me know when you've done that.

[#8]

Hi Joanne,
I've added it.
It may be that the new router fixes the problem, I just don't want to end up going through this again with a new router if it doesn't.
In the last couple of days I've cycled through every available version of firmware for my current router with the same results.
It's not a panic job, I still have surf level speed, it's just frustrating.

[#9]

I received and installed the new router on Saturday morning (Kudos to Plusnet, it was truly idiot-proof. Plug and play heaven).
It's doubled my speed. I now get an average of 1115 Kbps. 
I still get close to 160000 Kbps between 3am and 7am.
I investigated and the speed jump isn't at exactly the same time each night.
I had suspicions that there was something on a timer interfering with my line (a noisy electric heater for example).
Could another user near me be creating a traffic jam?

[#10]

Thanks feroluce,
I'll get this flagged back up to the faults team again for you.
Adam