Speed and Noise Margin

[#1]

I changed to the 'Pro' package around christmas and my kids saw a good improvement in game lag.
In January I was seeing download speeds of 5500 to nearly 6000 kb/s with latency of 26ms (speedtest.net),
now I am getting fairly consistenst download speeds of 3300 to 3800 kb/s and latency of 40ms.
My router currently reports a connection speed of 5280 down and 448 up, the noise margin reads 14 dB upstream and 23 dB downstream on 'Fast Path' link type.  The router shows a fair number of 'Interleaved Path FEC Corrections' and a few 'Interleaved Path CRC Errors', as far as I know, these are not relevant as my connection is not interleaved, the Fast Path errors all read zero.
Is my noise margin too high?  Would a reduced target noise margin improve my speed (to what I got in January?)?  Should I raise a ticket to get the noise margin target adjusted?
Thanks for any connents or suggestions.

[#2]

The noise margin is automatically set by the equipment at your exchange. It will go up if there are a lot of errors or disconnections on your line in an effort to make the connection more stable. PN can request that the margin is lowered, but BT are reluctant to do this more than occasionally.
I would suggest the first thing you do is read <a href="http://community.plus.net/blog/2007/07/02/broadband-speed-faults-how-to-diagnose/>this</a>. Have you changed anything in your setup e.g. plugged the router into an extension, installed a new phone, Sky box or somesuch? Try replacing the filter, as these can fail.
Also, try connecting to the test socket in the BT master socket to see what the connection figures are there. This will eliminate your internal wiring. Assuming you have a modern master socket, you can unscrew the bottom half and you will find the test socket inside, slightly to the right.

[#3]

Thanks Denzil,
I only have a master socket, no extensions, nothing new plugged in.
I will try another filter though.