Just a follow up on this issue with power. I've been running the circuit non-stop for a month now and the problems are gone. The combination of putting current limiters on the serial lines and enable brownout detection has kept the serial line from keeping the chip running with the power off.
Since the USB serial adapter I'm using is powered by USB, the problem may not exist if the serial adapter is powered by the circuit. All of my adapters are USB powered so I don't have a way of testing this.
Eddy
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