The other option is to maybe consider disabling Bitswap in both directions as some do think CRC are made worse by having bitswap enabled ?
This was true for the DSL-AC68U but that did use a MediaTeK DSL chipset which was hyper sensitive etc 
FYI – Bit swap could be seen as a first line of defence against brief burst type noise spikes. Without bit swapping most lines would see a lot more errors and loss of syncs. The tones where the most bit swap occurs should mirror the tones where SNR is lost from. For example when I had the line fault which affected my U0 channel, thats where all my bitswap was happening, and none in the higher frequencies. You can also use a bitloading graphs to see where noise has been problematic over say 24 hours
Issue is underlying line fault (or compatibility with IFTN tones) and/or noise (my view) not much @bully can do regarding old firmware on cards so we are just left with tweaking modem to get the best we can 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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