So I did some more tests, and I definity think it is some sort of short circuit, in stead of an internal MB problem.
Reason for this is that the problem started to get worst. At first it always felt at random, but it got worst to the point where it would happen when I tried running 3dmark benchmark, constantly.
By accident, I was annoyed after the a few attempts to sort it out, so after another trip, I shifted (i.e. bumped-shove) the pc case backwards to make room for something else (PC case is on the floor).
When I turned on the PC again and tried running a benchmark, it actually ran without problems. This made me suspect an actual short maybe somewhere.
Open upped the case, had a look around etc. and then tried running it with the case on my desk. I ran the benchmark a few times - no problems, then decided to do a 3dmark stress test - did it and the PC didn't short at all.
Next I put back the panels and stressed test again - once again no problems occurred. I did a few more tweaks after that and each time ran the stress test, and no problems occurred.
This in my opinion points to an actually short circuit problem, and not necessarily a MB problem (apologies ASUS). Now what exactly causes the short, I unfortunately still don't know - it could even be maybe a peripheral cable at the back being too tight due to distance?
So, I quickly made a little 'raiser bench\ for the case, to lift it from the floor, since I don't have space to keep the case on the table. I once again did a few stress tests and no problems have occurred (yet).
On the one hand I am relived, but on the other I would sure like to know what caused the short circuiting to happen, since it can still technically happen.