Lopey idle on startup, possible causes?

Yeah man, I had a similar problem with my 93 Gt. I have a TFS Stg 1 cam in mine and had the same problem. The thing is, the car is used to the old cam. Everytime you start up your car, it will do that for ahile. You gotta get someone to adjust your idle or you do it. This will compensate for the computer thinking you still have the stock cam in your car. Now I am not saying it will not go down and then come back up......the car just has to find the idle, that's all. After it sits there for about 1-3 mins, it will adjust on its own. Do not buy a chip, becasue that is what I was going to do until I talked with a Mustang Expert here where I live. Adjust the idle and please have someone that knows Mustang do it. Hope this helps you out.

you can adjust the idle screw on the tb until it clears up.

rough idle that doesn't go away is usually a vaccum leak, check intake manifold, and vaccum hoses for leaks.

Hanging idle is probably the IAC, also clean your maf wires specially if you ever had a K&N style reusable filter which has oil on it.

One vacuum line in particular can be a problem. The one that runs from under the intake manifold foreward and down to the vacuum canister. I had to replace it when I removed my intake manifold.

I had the same problem, the only thing that fixed it was adjusting the voltage
on the tps, I had it done at a mustang specialty shop never hapend again..