EmonTX stops working after a day (roughly)

I acquired a new emonTx v3 just a week or so ago. I’ve installed it very close to the emonPi (so tx distance is not an issue). It’s powered with the AC-AC adapter.
After roughly a day or 2, emonPi doesn’t report any data from it, and I have to reboot the emonTx to get it to send data again. Do I have a faulty unit, or is there anything else I can use to troubleshoot?

What do the mains voltage readings show, both from the emonTx and from the emonPi?
How does the emonPi’s voltage, when the emonTx fails, compare with “normal”?
Is there anything unusual in the emonPi’s logs at the time it stops?
Does it stop at random, or at a specific time, or might it be related to something else going on?
Am I right to think that you have the factory-loaded sketch in the emonTx?