had the exact same problem, turns out most of it was vacuum related and a dirty IAC valve. had cracked lines with a clogged pcv valve, as well a a filthy intake manifold that took about two cans of spray cleaner to get all the carbon out of the iac, intake, and the channels between the iac and main bore of the intake (they clog).
The biggest help of all was changing the vacuum modulator on the transmission. Turns out that the vacuum from the engine is used to help determine shift points in the transmission and when the diaphragm goes 2 things happen: your car stalls when cold and will eventually start sucking
trans fluid up the vaccuum lines into the intake. Neither of these are life threatening (unless you run your
trans dry) but stalling and adding
trans fluid all the time is a real pain in the a@@. It's on the passenger side of your
trans and has a outlet that ponts to the rear of the car before the vacuum line loops aroud and heads back to the engine compartment. It's also a pain to change, I had to remove part of my exhaust system just to get in there so you might want to get a shop to do it, depending on what tools you have, etc. That said, no more stalls and no more leaky
trans. The vacuum modulator cost about 25-30, a few feet of vacuum hose was under 10, and the two cans of intake manifold cleaner in a spray can were about 10 as well I think.
good luck
1989 Mustang LX, 2.3 Goes dead when cold and put in gear. 
Vacuum Modulator problem 
