I am having precisely the same issue. My phone is rooted but stock rom and not overclocked. It started yesterday when the phone would at first restart randomly then it sped up and now I can't get past the varying stages of starting up without going straight to black screen with constant haptic vibration.
I can get to the recovery menu but don't have time to select any option before it goes right back to black screen and haptic vibration.
It was weird that I hadnt made any software changes preceding this although I am on an international trip so I suspect some setting change might have brought this on. The alternative is a bad power switch button but I lean towards software issue.