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Unstable Rig (4x 5600xt and 3x 580)

Hi, I’ve added an extra 5600xt to my 3x5600xt + 3x580 which was running absoluty fine.
After the new adition, the rig became complety unstable, i am unable to do any kind of overclocking and that would result in the rig crashing every 20min or so.
What amazes me is the fact that the rig works complety fine without any overclock (only undervolt) and also wihtout undervolt (consuming around 1000W or more vs the 600W or so with the undervolted settings) so i dont think is a PSU issue.
What are the main causes of failiure in these cases?