That's interesting, and you're sure that #0226 is stable? There's not much change in the #0305 firmware, but popcornmix will know for sure.
Can you confirm the device you have - is it RPi2 or RPi3B? I'm assuming it's not a RPi3B+ based on the versions you have been testing. Edit: You've already said it is RPi2
Just to be sure there's nothing non-firmware involved, can you also test the individual builds between #0226 and #0305 (ie. #0227, #0228, #0301, #0302, #0303 and #0304) and confirm that they are all stable - if any of them start rebooting then it's not exclusively a firmware issue.
I appreciate this is a lot of tedious, slow work but hopefully it will prove fruitful in the end!
Mario77 if you still have your RPi2 are you able to confirm that #0305 reboots, and #0226 is stable?