The only reason I recall your stats is because at one time they were so similar to my own, but admittedly that was last year and things can change. So Ive just gone back through on MDWS to give you my analysis.
On the 17th of Nov, it looks like you may have picked up a heavy crosstalker, theres nothing much that can be done about that, but the line still remained in sync at 80/20.
Jan 08 it looks like the line started a slow degrade of SNR (possibly mild crosstalkers), the SNRm went down to 5dB, then 4dB yet the line still stayed in sync and the Err Secs were low enough so that the DLM didnt care. That suggests that the line was a very stable 80/20 but hanging on in there, but the next time you did a resync then you will have lost some of the 80Mbps.
I'm afraid to be the bearer of bad news in that crosstalk has cost you some speed and you may not quite get 80Mbps again, but that does NOT justify the 57Mbps sync in March and 60Mbps. At a guess you should still be reaching somewhere in the mid 70's but I haven't calculated it exactly, certainly no lower than 70Mbps.**
Something drastic happened on Feb 14th. Not only did you lose your downstream speed, but your Upstream SNRm took one hell of a hammering. Your Upstream SNRM dropped from 12dB to 6dB. You lost 6dB of upstream SNR overnight and that certainly isnt right. You probably wouldn't have noticed this because there was still sufficient SNRm to give you the 20Mbps.
There is also something strange occurring with your upstream SNRm. If you look at it over the period of the last 6 weeks it has some 'castle top' like activity _|¯|__|¯|_ down to 6dB, up to 15dB, down to 6dB up to 11dB, down to 7db and now back up at 15dB. That certainly is not expected behaviour and something odd going on for your upstream.
There are certain things I cant tell from MDWS and so can you please post me a full set of your current stats. (adsl info --stats)
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** ETA
I just thought, I hadnt included any compensation for g.inp in that calculation so in theory you should gain some of the loss back again, and if g.inp works as it should then you 'should' be in the high 70's