My 3D Games Crash after sometime when i start them? What should i do?

That definitely sounds like a video card or memory issue. Have you upgraded your computer's video card or memory at all? I used to have a similar problem running games with high textures and high graphic requirements. I then upgraded my video card and the games ran SO much better. I still have some lag so I'm planning on upgrading my memory to a 1gb so that should take care of that problem. My advice is to definitely upgrade your video card first and then your memory if you want to. Good luck!