lag after 25 seconds after game starts FPS go to 0

i have that problem since s8.I dont know what is wrong but this is my second pc and lag follow me.Its like i start game and in 25th second of game my FPS drop to 0 and it come back to 300 i guess it happend to me every single match i can play clean because my game is not clean let say i am soo high elo that my game dont allow me to show to riot plat-dia for sure