Update
I got a replacement cpu from Intel. Which worked fine for about a month. Now the same behavior is back.
The motherboard power cycles on and off with no beep codes (leds flash fans spin then stop). I have disconnected everything and reset the bios. I had the same mild overclock as before...
Everything has been done outside of the case with all non-essential parts disconnected. I have tried:
reseting the bios
re-seating the CPU
one stick in different slots and without (I haven't tried different ram)
different motherboards
I even tried the boards without CPUs (nothing happened)...
I've just checked with a new PSU and I get the same behavior. I also have attached a speaker.
The board powers on and the CPU error light turns on. There are no beeps and no video output.
So by processes of elimination it has to be the CPU.
I forgot to mention everything is still under warranty, so I'm hopefully I won't have to buy anything. However buying a cheap CPU to test with might be the next step.
So I have an Asus Z170I Pro Gaming with a 6600K.
The first problems I had was the PC would freeze randomly. I could not get a stress test to duplicate the freezing. This happen several times over a week.
I went to turn on the computer one morning and it would constantly power cycle on and off...