My friends 280z has this problem. When he's driving and the car is in gear he can push the clutch pedal down and the revs either hold or go a little higher. If he holds it down long enough they drop. We mainly noticed this problem at stoplightswhen its in first gear and he's holding the clutch down. My guess would be an IAC, but Im not sure. Any suggestions?

on some of the 280z's there is a large cylindrical thing that hangs off the bottom of the tb, it was supposed to keep the revs up between shifts or something it was only used for a year or 2, it coud be that and of so it might be malfunctioning