I have a 91 civic that has an idling issue. While the car is idling, the idle levels out and the car runs smoothly then all the sudden the idle drops off to the point that the car is running but the exhaust is rattling awfully. Does anone have any ideas as to why it may be doing it. No check engine light, plugs cap and rotor all look good, wires ohm out good and are less than 6 months old. Any help would be greatly appreciated.