Unfortunately, if the vehicle won't repeat the symptom in the presense of the dealer, there's not much they can do. If you're lucky, it will either do the deed for the dealer or they'll find a fault code in the computer when they hook up the diagnostic machines.
Either way, I agree that an 800 to 1500 rpm swing, whether your in park or not, isn't normal. I'd bet you've got a bad sensor somewhere.
Good luck!
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Drive Safe,
Steve R.
'01 SS LX
Cargo tray, leather steering wheel, mud guards, alarm, fog lights, transmission cooler, in-dash CD player, Kelton subwoofer, under seat storage tray.