have you already increased your virtual mem to at least 16GB or more?
For multi-GPU systems, set Virtual Memory size in Windows at least 16 GB:
"Computer Properties / Advanced System Settings / Performance / Advanced / Virtual Memory".
Yes I did. Also tried 48GB because I read somewhere you should take the same number as all your GPU ram's together (6 x 8GB).
To be honest, to me it still sounds like a PSU issue. Your PSU might not be powerful enough or maybe something is wrong with it.
Okay thanks, guess I'll try to power one GPU with the PSU from my normal PC.