2005 Subaru Impreza High Idle 2-3000 RPM. Dealer cannot fig

After driving medium length distances (around town, stop and go, or on highways) and stopping (for lunch for example) and turning off the car, upon starting again the car will idle anywhere from 2000-3000 RPMs. The engine revs high as though I'm pressing the gas pedal getting ready for a launch, but I'm not.
Dealer mechanics first said it was a clutch. Replaced that for $1500. Didn't fix it. Then they said it was the Power Steering Pump and Neutral Safety Switch, replaced that for almost $900. Still didn't fix the problem. No "Check Engine Light" codes show up, and the dealership can't figure out what's wrong with it. Someone suggested a leak with the turbo, or a bad alternator/voltage issue? The car is fairly new and has low mileage. And I can't fund the dealership's experiments any longer. Help!