One of them reboot the miner sporadically every 10- 20 minutes.
I’ve tried many OC Under volts with no success
2021-Sep-10 18:10:32: GPU 3 hung up
2021-Sep-10 18:10:32: Miner has not been restarted before
2021-Sep-10 18:10:32: Calling reboot script (./reboot.sh)…
What mining software are you running? is it always GPU3 that hangs? check that the riser card has not gone bad. I had a similar problem with a rig a while back that was restarting its self every 20 minutes and it turned out to be one of the riser cards. Try another riser with the GPU that is hanging
When it comes to finding problems in a mining rig cross elimination is key. It could be best to remove the GPU causing issues and see if the mining rig is stable without that GPU installed.