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Car revs up and down while idling

My 2001 Isuzu Rodeo Sport (65k miles, auto transmission) sometimes revs up and down (from 500 to 2000 revs/min) while idling in Park. When it does this, putting it in Drive causes it to shudder as if it's going to stall, but once it gets going, it runs fine.

This doesn't happen every day, but it doesn't seem to be related to the temperature. And it only happens after the car has sat overnight. I fear that this portends bad things to come. I imagine that the gears may be slipping, but am not sure. I have recently had a transmission flush, but that doesn't seem to have made any difference.

Any idea as to what might be wrong?


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