High Errors

[#1]

We had BT come out and do some work to get our connection more stable and considerably faster. (Up from 4Mbps to 15.5Mbps when he left).
The engineer was out due to high FEC/CRC errors and a drop in speed.
Our line was reset and settled at 12.9Mbps sync by the end of the 10 day training period.
The FEC errors did start up again but the engineer said not to worry too much so long as the line stayed stable.
All seemed fine until about 12:25pm today when the errors shot through the roof. FEC, CRC and HEC.
CRC errors are currently going up at around 50,000 every 15minutes. Having been told by the BT engineer that a few thousand over the course of a week is unacceptable...
Seeing this erroring I thought I'd try switching the router off and on again, however all this achieved is a drop of sync speed to 9.5Mbps.
What is going on?
Just after the engineer visit we had zero errors with the SN margin dropping as low as 1dB.
I've attached several screenshots on the router details page to show progression of errors.
I've also attached the line speed page from member centre.
I've been unable to perform a BTw speedtest as after leaving it running to attempt to complete after 20 minutes it timed out.

[#2]

Hi briarley17,
I've just taken a look at your line and I can see the errors that you're seeing too.
I've just passed your Fault Ticket back to our Faults Team for further investigation. They will be in touch as soon as they can.

[#3]

I didn't think this post had actually made it through the error and had just sat down to write a new one now the line has stablised.
As an update, we have now had our IP profile reduced to 6.53Mbps.
The CRC and HEC errors are gone.
After the engineer visted we were having almost no errors, and a sync speed of 15.5Mbps. Over the 10 day training period this dropped to 13.5 then 12.9Mbps, which was expected, with very few errors.
The SN margin worked its way up from less than 3dB to settling and bouncing between 6 and 9dB, again expected, and again with very few errors.
Attached is the details page again and a BTw speedtest.

[#4]

You've had quite a few drops yesterday;
<img src="http://community.plus.net/visualradius/generated/image14000628129353.png"/>
Your SNR margin on your downstream is currently at 20dB, we need to see if your line does remain stable over the next day or so in which time our faults team will have been in-touch.