Do not take me wrong, but coming back just 1/2 later asking again does not help.
People like to help around here, but no one likes to be pressured.
DO A SEARCH, probably you would have found all the info you needed in the 1/2 wait for a reply. There has to be ton of posts in the archives with your same problem.
Here is my 0.02 cents:
That same condition was caused by an O2 sensor going bad in my car. It could be something else in yours.
Get yourself a multimeter with duty cycle reading. I believe they are on sale right now at Sears. There is a post with a picture and the info.
I will be gald to help you with details on how to diagnose the car if you do not find it already posted.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car