New SD card arrived today and the unit is now working as expected. @PaulZC I think you are correct re brownout, thanks for the assistance.
Couple last notes for posterity:
- RTC was reset often/always(basically every time I got it working again)
- I was suspicious of the USB power (handshake or controller issues), so I tried running on battery only, but saw the same results.
- I also tried running it from a cell phone charger (again suspicious of the USB connection), also same results.
Finally the card that worked was a Sandisk Ultra 16GB UHS-I. The bad card was a 32GB Kingston Canvas Select Plus (also UHS-I).