Currently Being ModeratedMay 1, 2013 1:58 PM (in response to Timothy Arends1)
It's not true for an Apple Store, no, not if the system is under warranty or under an AppleCare extended service contract. At least I've never heard of an Apple Store charging such a fee. To the best of my knowledge authorized service centers are indeed compensated by Apple only for fixing problems, not for diagnostics that turn up no fault, so a service center may charge a diagnostic fee for situations where they can't find a problem.
Make an appointment at your local Apple Store and talk to them about the issue.
Currently Being ModeratedMay 1, 2013 2:10 PM (in response to Timothy Arends1)
Do note that since you have AppleCare's protection plan and if you're within 50 miles (80 KM) of an Apple repair station, you might be eligible for onsite repair since yours is a desktop machine (depends on where you're located—usually not a problem in the US).27" i7 iMac SL, Lion, OS X Mountain Lion (10.8.3), G4 450 MP w/Leopard, 9.2.2