My car seems to have a stumble when cold. It usually only happens when I am leaving my house in the morning, and until the car warms up some. Then after that, it seems fine until it sits over night again. It happens when I am starting from a complete stop/coming off of the clutch, and the car seems to jerk/buck. But like I said, after it is warmed up and throughout the rest of the day, its fine. What could be causing this?

 

I've had a similar problem with my saab ever since they started putting ethanol in gasoline. During the beginning and toward the end of the summer gas season when it's a little chilly in the morning, my car doesn't like to start up. Once running it might stumble once while it's still cold, but once warm it's perfect and has no issues the rest of the day. I chased this problem for 2 years and just gave up.

As you have a custom ECU I'd double check everything is kosher with that. Other than that, I've got no clue.