PC sometimes does not boot

Hello,

I got a really strange problem with my PC. The "sometimes"-problems might be the hardest to solve, but maybe someone has more insight than me here.
To better be able to understand the possible problems, I'll list the parts as accurate as possible:

PSU: Corsair AX1200W
MB: Asus Maximus VI Extreme
RAM: 32 GB G.Skill Ares DDR3-2133
CPU: Intel Core I7 4770K
Graphics: EVGA GTX 680 Hydro Copper Classified @stock
Cooling: Mora 2 Pro with 9 attached fans all connected to the PSU, 2 chassis fans, Leing water pump
Hard Discs: 2 SSDs, 5 SATA 2s
Keyboard: Logitech G19 (own power supply)
Mouse: Logitech G700
Monitors: Asus PG278Q ROG + 2 Dell U2711 + 1 LG 60" TV (all own power supply)
1 DVD writer: LG GH22LS50
Sound Card: Creative Sound Blaster X-Fi Titanium Fatal1ty Champion (lol)
OS is Windows 7 x64 Ultimate
Several temp displays at the front of the chassis.


Usually my PC boots up fine but sometimes (about every 5-10 days) it won't.
I press power, PC makes noises as if everything is booting up fine, screens stay black however and also the Chassis Fan 1. A few times though some fans on the Mora 2 also stayed still. One time the CPU temperature reported 0°C, so I presume the CPU did not turn on that one time? At all the other tries, the CPU had a reported temp.
When I hold power a few secs to turn it off and then press power again, to try it again, sometimes it works after a few tries!
Usually I try to change something after the fourth failed attempt though, so I cannot say what causes this.

Things I tried until now:
I reset the CMOS (until I did this, my BIOS clock was frozen too. Resetting the CMOS fixed this and the PC started correctly after this too - it could be a coincidence though.

Another time (this morning) I switched to my second BIOS on board. I immediately got a screen where I saw the BIOS was loading an update (probably from BIOS 1). It then restarted and I needed three more tries on BIOS 2 to get the PC to boot.

I switched the fan from the chassis slot 1 to the CPU fan 1 slot on the motherboard.

I removed the overclocking of the CPU (was at 4300 before (turbo off), now at 3500 (3900 turbo)).

The problem persists.
The weird thing is: apart from this, the system is 100% stable even when I let the PC on for more than a day. There are no crashes, no performance issues, no screen-issues at all. All temps are at around 24-34°C, even CPU, RAM and GPU.
There are several layers of over voltage protection on my house power supply, the PSU and the motherboard itself.



EDIT:
I forgot to mention since when these problems occur:
When I got a Saitek X52 Pro I had problems for 2 days to get it running. The system kept crashing and freezing and the reason was the "intel USB 3.0 eXtensible Host Controller" which I deactivated (or deinstalled, can't remember). As soon as I did that, the problems with the Saitek were immediately gone.

And the boot problem started occurring from time to time...
Now I fear those crashes broke something.