PinePhone quickly crashes unless plugged to power

[gab]

Hello,

I have recently changed the motherboard of my Pinephone Braveheart to switch to the 3GB version.
Since the change I have a problem: the phone crashes (instantly goes from running to powered-off) after a few minutes (between 10s. & 10 min).
If I leave the phone plugged to power (for example to a USB battery) it will not crash (not even if I use it to browse the web for 1 hour, or if I let it run idling during the night).

All other functions work fine.
The phone runs hot, but that was already the case with the original motherboard.
I am running up-to-date Mobian.
I am running from SD card, I will flash to the eMMC asap to see if it helps.
I had not this problem before changing the motherboard (crashes were rare, maybe once a month), but I was running from eMMC.

I tried logging dmesg -w to a file but this does not show anything conclusive.

Any idea for how to debug this ?
Should I try other distros ? How likely is it to be due to running from SD card ?

Thanks in advance