Let me jump in this question albeit belatedly. My mileage with the Hy6 / AFi in cold weather varies. At the end of February last year I took my Rolleiflex Hy6 (and film back) to the winter wonderland of the Swiss Alps. When I was shooting at over 2000m and with cold wind, I got the same behavior as the original poster exposed: the camera switching itself on and off, as if in a loop. Very hard to take images under those circumstances. In the end I managed a number of shots, while switching of the AF and in between reboots of the camera software. Afterwards the camera went back to DHW for review. It came back with its system software reinstalled (no further explanation) and I have not noticed this behavior again (but, admittedly, I do not recall having used the Hy6 body under similar circumstances again).
Now, the interesting part is that, in March this year, I was on a short trip to Iceland but this time with the Leaf AFi (and film back). At some point, again with cold wind and temperatures hovering below minus, a similar behavior showed up - the camera switching itself on and off. I did not send back the body for review and will ask the question to DHW.
Not sure if this can definitively be traced back to cold weather situations but it is something to be further investigated.