Stumbling issues on startup...have Durametric...where to start

[papasmurf]

For a long time now I have had an issue with my car ('00 2.7) upon startup when cold. It starts fine but after about 15-20 seconds of idling the rpms drop and it comes close to stalling but the computer catches it and it idles fine from there on out. It does this only one time when warming up from cold. I just got a durametric and was wondering what systems to test and check to hopefully find this issue. I took the car to an independent a long time ago and they did a smoke test to check for intake leaks and supposedly did not find any. My experience with this service place was not the best but they said they did the work and could not find any leaks but they suspected the AFM was bad. I bought a new AFM myself and installed it shortly afterwards and it seemed to make the car run better. When depressing the clutch once the car is warmed up, the engine note audibly changes like something is switching on/off or switching over. It is most noticeable when the car is cold and rarely I will hear a faint sound of a backfire/pop through the intake side. The AOS has been changed as well as the AFM, and the injection pump seems to run just fine as well if that rules out anything. There is plenty of vacuum on the oil filler cap as well. I have read the clutch and brakes share a reservoir so was wondering whether there could be an air leak or something that is tied to that. There is no leaking of fluid from the reservoir or any of the brake lines, etc.