@kopca
There seems two possible causes for the restarting symptom, neither very palatable and one likely fatal:
1. There has been incompatible/corrupt data indelibly written to the device non-vol region (rgn154) which cannot be removed by normal clearing of non-vol.
2. The flash chip is failing and will eventually die completely. If so it's toast, strip it for usable/saleable parts and junk the mainboard because replacing the chip and subsequent re-programming requires specialist equipment and skills.

If it's the first, there is still a slight change to recover it by erasing (not clearing) the non-vol. Use the usual SD method with this command "ergn,154" and "reboot". It that's successful the device will boot properly and partially re-create clean data in region 154. However, some functions may be lost and it may not properly identify the specific model on the splash screen. But it will work, mostly or even fully as intended.

If erasing doesn't change anything or results in an error then unfortunately it's almost certainly a hardware failure.