I'm fairly sure I've figured this out and come up with an easy fix
First the cause:
Poor fitting battery, made worse by a badly designed micro-SD slot position.
When you have an SD-card fitted this can push the battery away from its contacts. I found my battery had about 0.5 mm of movement up and down. Every time my phone reset it was when it was in my pocket and subject to movement.
Riding my bike to or from work.
Lovely and simple just place a bit of paper placed under the bottom of the battery to push it up towards the battery contacts.
I used a UK lotto ticket (loosing one) it was just the right thickness to stop the movement. (Tesco till receipts work just as well.) Tear your lotto ticket in half then half again, wrap your quarter ticket over the bottom edge of the battery, push the battery into the phone and check for movement. You may have to double up, I found just one layer was enough to stop the vertical movement.
fit the cover and turn on your phone.