idle trouble

when my car is in neutral it revs at around 800-900 rpm (normal). But lately, when i pull up to a stop sign or whatever and put it in neutral it drops to like 300-400 and sounds like it's about to stall..but then it'll go back up to normal rpm. Some days it'll do this and others it won't..anyone ever had this happen to them before? oh btw it's a 89 civic zc......