Thanks to all responses. Indeed it was insufficient battery power. While the batteries were "new" (i.e. never used before), they were also about 3 years old. There was enough juice to run the unit, but I guess when I tried to switch on phantom power, the batteries couldn't source enough current, so voltage dipped too low & the microprocessor rebooted from low voltage. (I don't have data to prove that, but I'm pretty sure that's what was happening).
Thanks again to responders.