I would guess leaking fuel injector. Can you test fuel pressure? Something else you can check (and it's easier), is your throttle body. Make sure it's clean, and the idle air control valve is clean and functional. It sounds to me like you have fuel leaking into a cylinder, or not enough air at start up.

Also for giggles, try to cycle your key on and off a few times. This will make sure you have fuel pressure at start up.