Could be your psu mate how old is it I don't know much about your psu to be honest but I was once told never skip on the quality of your power supply it's the most important part of your pc maybe it isn't stable enough to run the 3080 gpu
It's brand new, 80+ platinum certified apparently. Although clearly it could be faulty.
I tried the 3080 with all GPU and case fans at 100% and didn't get any crashes however the PC was audible downstairs at the other end of my house!!! GPU was sitting at 60C under full load rather than 80C. When I switched the GPU fans from 100% back to auto the system shutdown within 2/3 mins. So either the GPU is bad or the heat its putting on the the MB/chipset is so high its causing a thermal shutdown. Is it usual for a GPU to cause a complete system shutdown? Would this not normally just lead to crashes and blue screens?