It looks as if my first theory has bitten the dust, then. If the a.c. voltage is exceptionally low, then it’s possible that the a.c. adapter won’t supply enough power to support the radio transmission, and the emonTx will crash. Normally, it will reset rather than locking up, but that was a possibility.
Have you seen this? EmonTX v3 taking a walk - #12 by Robert.Wall
There was no clear resolution to that thread, but it could help as a list of things to check. Obviously as you know it is stopping, any log immediately before it fails could be helpful.