It's only with Phoenix miner?
What about claymore?
There's the problem of popcaps, but if you have newest drivers, and also with reduced TDP, its hard to blame popcaps
Looks like a app problem, I think all miners are not yet optmized to run new nvidia cards, but try claymore for some time and tell us if this problem persists
Your hardware is good, PSU, all system updates...
Same hashrate drop with claymore as well.
I have not tried other algos yet as I recall KawPow will use more power and definitely cause the temps to rise even more
Memory is trottling, it is very hot on 3080, 100+ degres.
I guess Alexgtr is right, temps in MSI shows 50 to 52 degrees Celsius but internal temps seems to be off.