Broadband download speed suddenly dropped severely

[#1]

Hi
My Plus.net broadband download speed has suddenly dropped to about 1/10th of the speed is was.  Last week I was getting around 2Mbps (and have been for years) but since Saturday 9th July my download speeds have fallen to around 200-300kbps.
My router is a BT Voyager 2091 and the stats are:
Statistics                 Downstream Upstream
Line Rate         512 Kbps 448 Kbps
Noise Margin         22.3 dB 16.0 dB
Line Attenuation 61.0 dB 31.5 dB
Output Power       15.1 dBm 12.1 dBm
The BT speed test has returned the following results:
Test1 comprises of Best Effort Test:  -provides background information.
Download  Speed
148 Kbps
 
0 Kbps 500 Kbps
Max Achievable Speed
Download speed achieved during the test was - 148 Kbps
For your connection, the acceptable range of speeds is 50-500 Kbps.
Additional Information:
Your DSL Connection Rate :1856 Kbps(DOWN-STREAM), 448 Kbps(UP-STREAM)
IP Profile for your line is - 350 Kbps
Current line speed:
    0.35 Mb (!!!!)
My current line speed has been steadily dropping over the last few days as it is normally around the 2Mb mark.
Please can anyone help me fix this or offer some advice. 
Thanks

[#2]

The stats and the BT test results give contradictory information.  Which came first?

[#3]

BT test first, then router stats about an hour later.
Just done another bt test and got consistent results to my router:
Test1 comprises of Best Effort Test:  -provides background information.
Download  Speed
307 Kbps
 
0 Kbps 500 Kbps
Max Achievable Speed
Download speedachieved during the test was - 307 Kbps
For your connection, the acceptable range of speeds is 50-500 Kbps.
Additional Information:
Your DSL Connection Rate :512 Kbps(DOWN-STREAM), 448 Kbps(UP-STREAM)
IP Profile for your line is - 350 Kbps

[#4]

When you did the first BT test your line sync rate was 1856 kbps downstream.  Something happened in the next hour which caused the broadband connection to be dropped and when your line resynced it was at the much lower rate of 512 kbps.  Your line has a lot of attenuation (nothing you can do about that) and the current noise margin is terrible.  If your line manages to remain stable now for a few days I suggest you ask Plusnet to reset the line.  If it isn't stable now, the question becomes why not?     

[#5]

Hi dmrogers123,
Your speed will be affected by the high SNR, this will have gone up because of the line being so unstable recently. I would firstly suggest troubleshooting your intermittent connection in order to try to stabilise the line. One your line is stable your speed would then improve.
Jojo

[#6]

Sorry what is "SNR"? And how should I go about troubleshooting my intermittent connection?  Will that be to do with my router or some external factor?
Also, I do not fully understand what the graph is showing me!
Thanks

[#7]

SNR - Signal to Noise Ratio, sometimes called Noise Margin.  The equipment in the telephone exchange (DLM) sets the signal to noise ratio on your line to as small a value as it thinks the line can stand (minimum 6 dB).  Unless you live very close to the telephone exchange then the smaller the SNR, the faster your sync rate and thus download speed.  Your router stats say your noise margin is 22.3 dB which is massive.
The graph is showing you that your line is not stable and frequently drops the connection so the router has to automatically reconnect.
Your intermittent connection could be caused by a faulty router but there are many many other possible causes.  The first thing to do is to find your BT Master Socket.  Unless it is one of the rare new ones with a built-in ADSL filter, unscrew the screws and pull out the faceplate to reveal the test socket underneath.  Plug an ADSL filter into the test socket and the the router into that.  You can plug a telephone into the other half of the filter because removing the faceplate should disconnect the wiring to all your internal telephone extensions.  You'll have to try to leave the router in that position for a few days to see if that improves your line stability.