I owned 2 86 cutlasses both with the 307 engine,1 ran great the other had the exact same problem you describe.First of all is your check engine light on? If it is it is most likely emission related .You can pull the codes to get you in the right direction.Mine I replaced the sensor carb, the egr,the thin vacuum line that ran from the map sensor, the purge valve and finaly the real problem was found.It was the tvs sensor that bolts in the coolant passage in the manifold. Even though it appear ok it had melted and was not sending the right signal to the obd system for warm up.After that was replaced it ran like a top! ronvb