Ford Repair: races when cold, ford tarus, weather conditions

Question
I have a 1990 ford tarus GL 3.0 and when I start it the engine will race for about 5 seconds and then stop for about 5 seconds. If I am stepping on the brake it is like it is trying to go but I am trying to stop. It does not do this when the engine has warmed up. What is causing this?

Answer
Hello and thanks for the question...I'll try to help.
 I have a few questions so that I can lead you to a correct and speedy diagnosis:
1. Define "Stop"? Does it shut off? Does it go back to a "normal" idle? What exactly does it do?
2. Has the engine recently been tuned up?
3. What is the current milage on the vehicle?
4. Does outside weather conditions have an effect on it?

Please take some time and give me as much information about the problem, it will help a lot.
Thanks,
Erik