I have the same issue with my iPad Air 2 it would seem the more I try the more it fails. If I just let it sit for a while after reboot it will work. I thought for a while I had a tweak installed that was causing it but that doesn't seem to be the case. Once the JB works don't reboot.
It jailbreak first or second try each time for me (maybe max 3 tries sometimes) on my iPhone 6s Plus and iPad Pro 12.9 and when it's jailbroken it's very stable, no reboots or random resprings. I don't have many tweaks through (less than 10)