1992 Prelude idles inconsistently

We are repairing a 92 prelude and we bought a used motor with 20,000 miles on it and when you start it it will rev to 1800 rpms and then drop to about 1000 rpms and it will not stop doing it. You can drive it and when you are accelerate its perfectly fine runs like a charm then, but if you have to stop at a stop sign it sits there and sputters so badly we have tried everything we can think of in autos because it is our senior vocational autos class project so any info will help us greatly. Thank you very very much!