The weirdest trouble I have ever had. (Random shutdowns)

Hi guys, and thanks for checking this post.

I seriously need someone with more experience than me with this, since I really cannot figure it out.

I'll try to give you every detail and summarize as much as I can.

Everything started a couple of years ago. 3:00 am, watching youtube, the pc suddenly shut down and turned itself back on (no bsod, just black screen, and power on after 3 secs or so). After the "I-wanna-die" phase, I kept using the pc without issues for almost 6 months, so I thought that was just an occasional issue.

Then, I bought a new monitor with a higher resolution, so I added another GTX 970 with my old one. I started playing Battlefield 1 and after 2 hours the computer shut down (and back on). I said, well, it's easy, my old psu (Corsair RM 750) couldn't handle two GTX 970s. But still, the computer worked fine for 2 weeks. Then, it shut down (and rebooted, as always) again. This time, I wasn't even playing videogames, just messing around on the web. And the day after happened again. Desktop, again, no games involved.

After, maybe, 15 shutdowns in a row, I got mad and thought the problem could be related to PSU, motherboard, the 2 video cards, RAM sticks and the cpu. It was almost impossible to troubleshoot, in my opinion, so I've decided to replace everything I've listed. I basically built another computer, using the old case, cpu cooler, dvd drive, SSD and HDD drive.

As you can guess, it worked flawlessly for 6 months, then, again. First time on 16th of December, second time, 24th of December. After that, shutdowns became very frequent, like once a day (and this time, it NEVER happened while playing videogames, just surfing with Chrome).

I started my "Google -PC shutdown randomly and turns back on" odyssey again, and I found out that:
1) Dust can cause shorts (I cleaned my pc, and the issue was still there--->FAIL)
2) Case buttons can send wrong signals, so I unplugged my power and restart button from the motherboard and started booting my pc by the "Start" button on the motherboard itself. The pc stopped shutting down.

I was so happy, I thought I FINALLY found the cause (actually the case was the same of 2 years ago, so that made sense). Everything worked flawlessly for 4 months, and last night it shut down again.

I always try to figure out issues myself, but this one is just impossible for me.

Thinking about it, my statement here is:

1) Why the hell everytime I change something the computer stops acting this way, then it starts givin troubles again after MONTHS?
2) My two PSU's shouldn't be faulty, since they handle heavy loads flawlessly, why the hell shouldn't they handle idle situations?
3) I'm thinking about shorting-static issues, but I'm definitely not an expert here.
4) H100i can be guilty as well

2 years ago specs

CPU: Intel core i5 4690K (removing OC didn't solve the issue)
RAM: CORSAIR vengeance DDR3 16 GB
VideoCard: GTX 970 (2 cards last year)
Motherboard: Asus Z87 PRO
Cpu cooler: Corsair H100i
1 Samsung SSD
1 Western Digital 1 TB HDD
Asus DVD RW drive
PSU: Corsair RM 750
Monitor: Asus IPS 1080p monitor

My actual setup:

CPU: Intel Core i7 6700k (OC doesn't matter, sadly)
RAM: Gskill DDR4 3200 MHz
VideoCard: GTX 1080
Motherboard: Asus Maximus IX Hero
Cpu cooler: Corsair H100i
1 Samsung SSD
1 Western DIgital 1 TB HDD
Asus DVD RW drive
PSU: Corsair AX 860
Monitor: Asus PG348Q

I will appreciate every clue, since I don't have any.