3080 and 3090 need fresh air on the backplate.
Add some fan to blow the backplate, it is simple and will really help
I do this on my 3090. If not it throttle to 90 Mh/s after some minutes, but with backplate cooler, I run at steady 112 MH/s for 295W
Even when doing that the memory still runs at +95C on the 3080’s .
I had the room at 18C , cards running at 35C and memory was still reaching 90-95 depending on the card . I have a fan blowing on the backplate for all the cards