For the records or for any future question like this coming from other players, I have "hopefully" found the source of the problem.
My Gigabyte rx 570 GPU is a version clocked from factory @1280 mhz instead of 1244 Mhz of the AMD reference. For some reason it seems that in this new configuration the Game cannot work at this frequency. Setting the card clock frequency to 1244 Mhz as the reference one "seems" solved the problem, but i will keep testing..
The strange thing is that this happen only with RF4 , maybe a Unity engine issue ?