My FPS decrease after some time playing

Hello, I have this problem since some months. The thing is that after 10-20 minutes of playing my FPS start to decrease and also with some freezes of 1 sec that makes me fail a lot. I have a bad PC but anyways I had good FPS before and I could play a lot of maps with considerable FPS, now is unplayable after some time.

Suggestions? And thanks for read.

Just clean ur pc inside and also ur hdd with ADVcleaner, malwerebytes. There is also some paste that u should change on CPU and GPU after some months. That paste passes the heat to radiator so the cooling is working.