I just wasted 3 days on installing hardware and software. I mean totally wasted! I ended up with the exactly same setup after 3 days.
I had a GFX card doing weird things and rebooting my 7 year old slave PC in the middle of rendering. First I thought it was a GFX problem but later a motherboard problem. Many things I learned in that process.

It’s hard to find a used motherboard with LGA 1155 CPU socket. You can’t flash a motherboard bios without the CPU on it is supported. Installing a new motherboard on a already installed windows 7 OS is not going to happen. Installing windows 7 from a CD require additionally a 1.7GB update download and 200+ update installations. Some motherboards doesn’t know what to do when a second GFX card is installed and goes black – even though it should support 2 GFX cards. If its old shit you are dealing with – read on the internet what flaws it has before buying/installing it.

I could also say fuck it – and buy new hardware. But windows 7 doesn’t run on new hardware. And why does it have to be Windows 7. Because Windows 10 Windows Display Driver Model (WDDM) steals 20% of your Vram. Something that Microsoft didn’t tell you about!

So I dusted off the old motherboard I though was broken. Cleaned it with alcohol, changed some wires and used different PCI-E sockets on the PSU – and the damn thing runs fine again. So maybe next time I should start by cleaning the hardware first. Just hope it doesn’t start doing weird things again – but so far so good.