a4 issue

what happeneds is that when i go to start it it will start however after 3 seconds it will just shut off on me. when i pop the hood and play with the throttle to try to warm up the engine it will stay on however it will seems to jump up and down in rpm. when i put it into gear it still stays running however when i drive, it jumps from driving forward to it being in neutral, but i still have my foot on the pedal. i dont think it is the battery or alternator becasue i can turn on acc. and all the electrical aspects work, what can be the problem??