Why is my car reacting so bad to cold weather?
Plugs, wires, distributor cap and rotor, fuel filter, air filter? If all of those are fairly new within the last 20,000 miles then it could be sensors. But when you put it in neutral/ push the clutch in, the engine slows down and the pressure in the fuel system and air flow drops. Does it to it more when the engine is cold or warm? Does it die or just feel like its missing? Could also be your idle adjustment. If its idling too low you may have that issue as well. I had an older mustang that had an idle adjustment that did that. Once I adjusted the tension via the adjustment screw it fixed the issue. Not sure how the 99s are for idle adjustment. It may be electronic. Just a few things to consider.