Intermittent boot failure p- why?

I get intermittent boot failure with my desktop PC.
It's a high-end one for audio recording, Intel i7 based running Windows 7 64bit.

About once in every six or seven boots (sometimes less frequent - no pattern) my PC goes through all its boot checks on-screen and then hangs just before the Windows sign-in screen appears. A white cursor is left flashing in the top left hand corner of a black screen. Another variation is that the PC will, part-way through booting, switch itself off and immediately back on again, often resulting in the hanging boot. I usually have to manually switch the PC power off and back on again to reboot again, sometimes five or so times, before booting into Windows finally happens.
At other times the PC will boot with no problems whatsoever every day for days...

I thought the problem might be the SSD - my system disk - an OCZ-VERTEX3 -but last week I updated the firmware on it to the latest version and this hasn't cured the problem. Could it be the Motherboard than can cause such things? It's an ASUSTeK P8Z68-V PRO, and I have never updated anything on the motherboard since I bought the PC in 2011.

 

There is no common cause for this one.
I will suggest some things in hopes one of them is it. Try one at a time.
1) unplug any USB devices except mouse and keyboard while booting, if your motherboard has special USB connections for mouse and keyboard use them
2) update your motherboard BIOS
3) run Prime95 overnight to test your RAM
4) if you have a HDD, image the SSD and restore to the HDD, boot with the SSD out of the system
Sorry I don't have a single answer, post any cards and peripherals in your system if these suggestions don't work.