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Manual shifting conversion would probably be an improvement-Especially if you are willing to stay on top of the inevitable shifter / gear cable maintenance it quite regularly needs (pita).
better still would be to move away from Nuvinci altogether.
If it were me and budget allowed, then I’d upgrade to Rohloff in a flash. Otherwise whatever internal gear shimano hub people are using for cargo.
yes, it's still under warranty. Been reading others with similar issues online. Several of them saying there are known (by manufacturer) issues. Some have opted for manual conversions. Also bit of a faf as dealer (the nearest too) is 12 miles away and I'll have to break down the fixie to get it in the cargo area to ride home on & also have to start using myold deisel van! :-(