I can exclude all the problems above. I'm not using a riser, my PSU is fine.
Right now i'm having over 27MH/s in the benchmark (nicehash) but when mining it goes down to 24MH/s. Sometimes it goes for 27MH/s for a while but then drops again for a long time.
I think it's the problem with DAG file size or i have some kind of virus in my PC, because i don't see any other sensible explaination.
In 2018 i was mining with GTX 1070's so i have some experience. But i never had Radeon GPU mining, so i really need someone who is currently mining with RX 470-580.