Why there are jerks during gameplay (Athlon 6000+) ?

My pc configuration is

Amd Athlon x2 6000+
3gb DDR2 Ram
Ati Radeon HD 3650
Windows xp sp3

I am getting acceptable framerate of about 35 fps in games like crysis , callof duty 4 etc at high settings
But I am facing sudden jerks during gameplay . I am facing this problem not only in next-gen games but also
in very old games like "no one lives forever" So in short in all 3d applications . So system requirements is not at all problem .
Can anybody suggest what is causing this problem