Most likely a vacuum leak. Check for cracked hoses. If you can get it to idle in park, do so, open the hood, and start pouring water on the throttle body where it connects to the intake manifold (don't douse it, just keep pouring enough to keep it covered). If this affects the way that the engine idles, then there is a leak in one of those seals.
It could also be the intake manifold gasket is leaking, which is problematic in these cars.
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Wednesday, March 9th, 2011 AT 1:31 AM