Update on random freezes:
Yesterday I checked my RAM with Memtest86+ both with all RAM modules at once as well as all modules individually. There were no errors found, but I will let a test run for at least 20 hours today to be sure there's nothing wrong with my RAM.
It seems that the ethernet kexts are NOT the problem. As mentioned before, I removed the specific kext for the ethernet port (AppleIntelE1000e.kext), but this didn't fix the problem. Afterwards I removed the IONetworkingFamily.kext, but I still got a kernel panic.
What's interesting is that this time I got more information on the panic: it was caused by the graphic drivers NVDAResman and NVDAGK100Hal.
So I switched to the internal HD 4600 and let my poor GTX 770 sit in the corner, let's see if this fixes the problem. I also put IONetworkingFamily.kext back into the system so I can enjoy some cat videos until the PC freezes or panics - I really hope it does because I don't want my GTX 770 to be the reason.
This post was just to keep people with the same problem on track, I hope this contributes to a solution.
Little summary:
What is NOT the problem: audio drivers, networking drivers
What MAY be the problem: bad graphic drivers (NVDAResman and NVDAGK100Hal), bad RAM, motherboard malfunction, maybe something with the VRAM of my GTX 770
Edit: Phew, still got a kernel panic with the HD 4600, therefore it isn't caused by my GTX 770. RAM testing will run until tomorrow, when it's finished I'll report back.