Searched but couldn't find exact issue with my 86 firebird 2.8 automatic. Right when I start the engine (and also when driving and coming to a stop) the rpms will dip down to 500ish or maybe lower. Occasionally it'll stall. Rpms only drop momentarily then come back up to normal (probably 800ish). Runs strong otherwise... Has new cap, rotor, plugs, wires... .. Seems like something with ignition.. distributor.. maybe? Anyone have any ideas?

 

Still no response?... Just installed a new IAC valve

 

That would have been my first thought. Usually while on the throttle even a little the IAC is held open 20 cts or so and comes down to the idle setting gradually when stopping. As long as the vehicle is moving the idle is a bit higher until full stop. You might have to get a scanner to see what's going on. Might put the trans in neutral while still moving and see what happens when you then stop.