Thank you for the replys.
I've tried this fastboot cache erase .. didnt work
ruu is quite hard to find for Rogers HTC one (so far no luck ../ found some old ones links broken ... will keep looking)
I also got a chance to try and flash htc stock sense 5.5 with 4.4 kitkat from here:
(durm it .. can't post outside links yet sorry ..)
Phone still reboots before fully getting to os.
Weirdly enough, if I turn it off during this cycle. It turns back on by itself too.
So far to stop the loop, I have to boot to recovery (TWRP currently) and lock it.
I am guessing I should find a way to S-ON then flash an older rom.
Anything else I could try?
Thank You everyone.