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