I have the same issue as you, with an RTX 3080 Gigabyte Eagle OC. Starts out at 88Mh, slowly dropping to 77Mh.
I installed the GPU straight on the motherboard since this is also my gaming rig. I noticed that the reason for this is that the core clock is dropping automatically to around 1000Mhz for some reason.
Also, total power consumption according to Phoenix miner is around 200W. (same thing with Claymore btw)
Did you find a solution for this problem? Thanks
After a few days of mining the clock drops further low to 800 and even lower. Just pathetic cooling.
Some say replacing the thermal pads worked for them but this will void warranty.
https://bitcointalk.org/index.php?topic=5287329.msg55545084#msg55545084Here's a thread about similar issues with other 3080s.
https://miningclub.info/threads/vse-problemy-rtx-3080.70379/post-1840603