Thanks again, guys. You are very helpful! I haven't much time to posting a big reply and I only want to show what I noticed. In the screenshot you can see my GPU log logged by a program GPUz. I noticed that when FPS is low (8-30 FPS) GPU Core Clock is 50.6 Mhz and when FPS is good (55-100 FPS) GPU Core Clock is 405.0 Mhz. I searched in google about this problem a bit, I found out that there can be leaks or problems with my GPU Memory/Core Clock or something like that. This is my quick note about GPU and I don't know if it means something important.
essenbe and Dave76, thanks for advices and suggestions. I will actually answer your questions, post needed screenshots and update my system specs! I will do it soon (probably tomorrow). I just wanted to give a quick note about my GPU. Maybe it's worth something, I don't know.
Also, my PC started to lock down when I don't use it and when it is on idle. Maybe this is related to my problem because yesterday there was no lock downs.
I will update needed information soon. Thanks for the help!