Unstable ADSL since line upgraded to 20Mb download

[chez]

The ISP upgraded the broadband service about a month ago, and since then the line has become very unstable.
At first I thought that the ocassional dropout was the equipment finding a good balance.
But the service was practically unusable on Saturday afternoon with the carrier signal down almost as soon as it connected.
The ISP and BT are investigatng the problem, but is there anything I can do to improve things?
(The exchange is about 200 yards from the house, so initially at least it was running at 20Mb down/1Mb up.)
I have two safecom swart2-54125 running v2.97.

Current modem status:

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Modem Status
	
Connection Status 	  	Connected
Ds Rate (Kbps) 	  	18221
Us Rate (Kbps) 	  	444
DS Margin 	  	9
US Margin 	  	-23
DS Line Attenuation 	  	12
US Line Attenuation 	  	2
Trained Modulation 	  	ADSL_2plus
LOS Errors 	  	0
Peak Cell Rate 	  	1047 cells per sec
CRC Rx Fast 	  	0
CRC Tx Fast 	  	0
CRC Rx Interleaved 	  	2066
CRC Tx Interleaved 	  	0
Path Mode 	  	Interleaved
DSL Statistics
	
Near End F4 Loop Back Count 	  	0
Near End F5 Loop Back Count 	  	0

Thanks in advance.

[thechief]

Post the system diagnostics. Also connect via telnet/ssh, and post the output of "dmesg" (please put these things in a text file, and attach the text file - but first remove all personal/sensitive information before posting).

[chez]

Please find attached the two files requested.
I am not sure what the dmesg file will tell you.
Please tell me if I need to reboot the modem/router to get usefull information.
Thanks.

[thechief]

Ok, the dmesg output is totally filled with sync messages, and so it is not very useful. The problem generally seems to be with your line, or, more specifically, how you are running it. This is the clue

US Margin: 4294967273

The value is nonsensical. My suggestion is to run the modem at a lower speed. Select "ADSL_G.dmt" as the modulation, and see if that gives you a more stable connection (the speed will be halved). If it gives a more stable connection, then you can try increasing the speed. Select Multi_Mode as the modulation, which should default back to Adsl2 plus. Then trade speed for stability by running this code from a telnet/ssh login session

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setenv DSL_FEATURE_CNTL_0 0x03000000
/sbin/reboot

After the router reboots, you should get a more stable connection, but with a slightly reduced speed. If it doesn't help, then you can thus delete the setting

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unsetenv DSL_FEATURE_CNTL_0