The situation is, I have good fps , but sometimes the screen freeze for 1 second or half second, some frames are skipped, then back to normal.

Spec:
Mother Board: ASUS P5KPL-AM
DirectX Version: DirectX 9.0c
Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 3
Processor: Intel(R) Core(TM)2 Duo CPU E7200 @ 2.53GHz (2 CPUs)
Graphic Card:ATI Radeon™ HD 5750 Graphics
Memory: 3072MB RAM