Problems restarting my system... Any Ideas?

(Apologies in advance if ive posted in the wrong section, please feel free to relocate)

Hey, ok so I have a problem with my homebuilt PC, I've had the PC for roughly 3 years now and over the past year or so its been running anything but smoothly.

I have managed to fix a large amount of the problems I've encountered lately but theres one that stumps me and I've trawled the vast plains of the net with no luck either.

My problem is that I am unable to restart my system, From the OS I choose restart button the PC shuts down, Boots and hangs on a black screen without getting to the BIOS and it doesnt beep. I Restart after auto updates same thing happens, I restart with the button on the front of my build or the jumper on my MoBo and the exact same thing happens, it just hangs on a black screen.

I also have to manually refresh the CMOS 'Everytime' I boot or it will just hang on this black screen, but when I do this the PC will boot fine and will run without any problems at all and I've ran it for days on end and its fine.

So far I've managed to rule out the PSU, MoBo, RAM, GPU and HDDs/OS the only thing I havent been able to check is my CPU but like I said when the PC is turned on it runs fine and stable so Im doubting the problem is arising from the hardware.

I've re-built the PC many times and am fairly sure all the wires are in their correct places etc and Its just stumped me. I was wondering if anyone else has came across this problem and worked out a solution as it is very frustrating not being able to access and save modifications in my BIOS because I am unable to OC or run my drives in RAID anymore.

tl;dr -> I need to refresh CMOS EVERYTIME I start my PC or it wont boot. help?

Is there an Overclock on system? If so remove.

Have you done a memtest on it? If not, dl memtest86 and run it overnight for at least 8 passes.

Are all drivers and windows up to date?

How have you ruled out the individual components? Do you swap then into a working PC or did you swap in working parts from another pc?

Try memtest first.
If it's not RAM i'd go for mobo and bad OS.

Testing would be backup all important info, then reformat and reinstall OS.

For Mobo, would really require using a new one or using the current mobo in another good system.