You either have a power imbalance or a failing PSU. Do some troubleshooting
to figure out which. Swap the mobo power with a GPU and see if it's still the PSU
powering the mobo that gets hot or the one you moved to power 5 GPUs.

My guess is you're probably using unpowered risers so the mobo is powering all the
PCIe slots creating a huge load on the PSU powering the mobo.