FPS drops between 5 and 10 minutes in game?

This hasn't happened before and it's lowering my chance of doing something usefull in games. Usually i get around 60-140 fps in-game but for some reason out of nowhere, between minute 5 and minute 10, my FPS drops from high to very low at 1 FPS ?! it lasts for about 15 to 30 seconds and it's impacting my games! this hasn't been happening before and i've done everything i could to get rid of the FPS drop, i even lowered down the Graphics settings to very low and it still keeps happening. Anyone else getting FPS drops at the same time in-game? thank you Riot in advance.
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