If knocking around under the hood...check out the wiring harness...the main harness that connects to your plug wires and such...common problem on these cars...and in my case it helped cause a misfire issue...if the insulation is crumbling badly...prepare to pony up some decent money for a replacement or plenty of free time to rewire it yourself...and yes your idle issue sounds like a vacuum leak...pretty common save time and let an indie do it...or get yourself a mityvac and have at it...that tool will come in handy in the long run...

best of luck

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2004 Chrysler Crossfire