First runaways don't stop, they continue until destruction. To answer your question, there are several things that can cause that. The most common is the fuel system having a little air. When you first start it does it start then die then start a little hard? When you start to drive, it will get that air bubble to the IP, speed up momentarily, then go back to normal. It will also do that right before it runs a tank empty. Any time the engine starts to run lean it'll speed up then go back to normal, or just die for lack of fuel. There is also a fast idle solenoid that (if you don't set it before you start it) will speed the engine up a little when it's cold.
