Problem on running duke nukem forever?

i downloaded duke nukem from razor1911 and when i try to play it, it gets stuck and is not smooth at all. i have a 9600gt and if i could play crysis 2 then why cant i play this? the installation was a bit unusual but i put fraps and it shows around 75 fps but still has those moments where it becomes zero. can anyone help me? is anyone facing the same problem?