strange fps drop

[Cvan]

after some minutes of playing the game i get weird fps drop. it just keeps on being at 25. it doesnt get lower than that which is weird. i tried lowering the graphics, sadly it didnt help. it just keeps on being on the damn 25 fps until i zoom in and wait for fps to raise then zoom out back. i dont know what should i do. i tried lots of stuff but it didnt work. please if anybody has solution it would mean really much to me. thanks.