So the first step I suggest is to connect up an RPi to the serial (independently power the RPi) and collect the serial data and send to emoncms with emonhub.
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If that is reliable, then the sketch itself on the emonTX is not the issue, the ESP is (which IMHO is more likely). If you feel like trying to discover why, you can then setup a debug and have a go at finding out what is causing it.
I don’t know, but connecting up an RPi gives you another means of flashing directly to try.