What operating system are you using on your rigs?
Have you independently tested the card in question?
Windows 10 fully updated. tested card independently and it works just fine. Seem to only like being the only card on a mobo. Strangest thing.
There should be problem in 2nd psu.. Do you have updated drivers?
Yup all drivers latest version from Nvidia. Weird thing is it works on its own so not sure it can be a driver issue and I think its more related to a PSU or power delivery issue.
have you checked your bios is at latest version?
have you enabled 4g decoding and set the pcie lane speeds to gen1?
Latest BIOS installed with Gen2 and 4G etc.