My '89 B230FT has started stalling out in the morning after throwing it into gear. This only occurs with a cold engine. A second start will usually keep it running, but sometimes a 3rd is necessary.

I checked my IAC. It looked fairly new, but the previous owner of the car put it in backwards. I cleaned it with Gum Out and put it in correctly. Before reconnecting the hoses I just connected the wires. With the ignition on I could see the input port partially closed by the internal plate and the whole unit was vibrating (pulse modulation?). The next morning on a cold start the problem was still there, no improvement.

For what it's worth, I also removed and cleaned the breather box (the non-flame trap type). All recirculation hoses are clean. I replaced most vacuum lines within the last year, but that's not to say there couldn't still be a leaky one.
The Fuel Pressure Regulator is probably the original, but the engine responds to pulling the vacuum line off of it.

Where should I go next? What else could effect cold idle?