When you say reflash, do you restore your setting from a backup?

I mean, try to reflash and do nothing else, no restore, no CSSU no nothing. Let it idle like that and see if it still shutdown.

If not, it is something else you are restoring/installing/modifying.

Also, if you have a SDCard, try without it.

And maybe, you said your flashed the devicxe, btu did you do both emmc and rootfs? if not, try to do so.

And, i remember an old thread that i can't find again about someone who had a similar problem and it was just the battery connector on the phone which had bad contact, or the battery was moving just a little bit but enough to cut the contact.