Hey guys. I had the same issue using a custom build image.
I was googling with no sucess as well but signed up to this forum when I found the fix.
I was the same, a new image of picade would work fine without rebooting but others would reboot after 20-30 seconds so I knew it was software and not hardware.
I also noticed that it would only reboot if the picade was powered via the picade x hat and not directly via the pi.
However, it seems to be down to some options specified in the /boot/config.txt
I compared the two files from the base working image and the custom image I was using.
There were a few differences.
dtparam=i2c_arm=on was uncommented in the failing image but commented in the working image. I beleive this can mess with GPIO pins so I commented it. I think this is the most likley culprit.
There were a few others but I think thats the most likley. If in doubt, just copy over the working /boot/config.txt from a working base image after you have installed the xhat drivers on it.
My PI now works fine without rebooting using the power on the xhat.
I hope this helps you and others googling.
- Fazza