FPS Drop

My computer features:

Processor : Intel(R) Core(TM) i5-2450M CPU @ 2.50GHz
Video Card : NVIDIA GeForce GT 525M
Video Card #2 : Intel(R) HD Graphics 3000
RAM : 8.0 GB
Operating System : Microsoft Windows 7 Ultimate Edition Service Pack 1, 64-bit

Game Graphic Settings :

Resolution : 1366x768
Color : 16 Bit
Texture : Low
Bullet Trace : Middle
Bullet Effects : Middle
Bullet Trails : Yes
Blood Effects : Yes
Weather Effects : No

Why fps constantly falling?
Please Help Me