I must say, I have *exactly* the same.
I got a brand new, stock, M8. Rooted it. It ran fine.
Then installed xposed (using TWRP) and the random rebooting started.
Tried rebooting in safe mode and doing a cache wipe, no effect.
Even after un-installing Xposed, the rebooting continued. Only a factory reset fixed it.
After the factory reset, I re-installed all my apps - phone ran fine.
Only after installing Xposed framework (no modules!) the phone starts to random reboot - again.