No - I was using NI-MH1300mAh 1.2V (they were not run down). It also occurred when cable-powered in the car. I have now put in alkaline and the problem seems to have stopped, though this may be just coincidence. I contacted Garmin and this is their reply.
"I recommend you try a master reset on your unit. To do this press and hold
PAGE + ENTER + ON. Please note this will reset your unit and wipe all data."
I have not done this (yet).
I have noticed in other threads that many others are having power-down problems. Some are related to Lithium batteries (over voltage problems) but others do not seen to have a pattern.