Huge perfomance issues on night races

I use an I79700+GTX1660S on 1920x1080 resolution. V sync = on 60. My game is installed on HDD. Everything is at maximum settings (ultra +). In dry races everything goes beautiful. But in rain condition and night races I have fps drops (52-55-58) in some corners or other segments of the track (where the scene is more complicated - i think). BUT if I change in graphic settings only crowd+mirrors+trees from ultra to high everything goes stable at 60 FPS. For night races (without rain condition) I change the resolution from 1920/1080 to a lower one and after, I rolled back to 1080p - like an refresh and the game runs ok at a stable 60 FPS (maybe in some rare situation I have again FPS by 57-58 for 1-2 sec.)