PC is restarting on its own randomly

[Necko]

Hey there!
I have a problem with a PC. It seems to shut down and restart randomly, but most of the time when browsing the web. The PC just shuts down on itself and immediately starts again. No blue screen or anything else in between.
Here are the specs:
CPU: Intel i5 4690k, Delidded
Mobo: Gigabyte Z97-HD3 rev. 2
RAM: 16GB 1600MHz DDR3, 4x 4GB
GPU: GTX 1650 super
Hard Drive: Samsung 860 Evo 500GB
PSU: CoolerMaster 700W

The PC is mostly build from used part, some of them from my old PC. The CPU and the Mobo I was gifted by a friend. He didn't have any problems with those, but he did not use them in a while. For delidding he didn't use liquid metal, but thermal paste, which I replaced after many other tries to fix the PC.
Here is everything I've tried so far:
I did use other RAM sticks in dual channel, no success there.
I ran multiple stress tests to determine which part is the one causing problems. But neither a CPU stress test (Cinebench R23 for an hour), GPU stress test (Unigine Heaven), a RAM stress test (Heavyload for an hour) or all of them combined caused a crash. 
None of the components overheat.
I also tried updating the LAN drivers, which didn't help, and deinstalling them using the windows device manager.
Windows is freshly installed on the hard drive and every driver is up to date.
I also used another PSU, no progress here either.
Updating the BIOS version from v F9 to F10c, the latest one, didn't help too.

About 90% of the crashes occur, when browsing the web. It doesn't seem to make a difference which browser I use. I tested Chrome, Firefox and even Edge. Edge seems to do the worst nearly immediately restarting the PC. The others I can use for intervalls between instant crash and half an hour. 

I am kinda bummed out now and don't know what to do. If I had to guess the Motherboard seems to be the problem, but I don't want to buy a new one if something else could be the problem.
Can anyone help or suggest something for troubleshooting?

[dilpickle]

39 minutes ago, Necko said:

If I had to guess the Motherboard seems to be the problem,

Good guess.