The idle of my '01 C70 convertible cycles between about 600 and 800 RPM when the car is in park or neutral with the a/c off. If it's in gear or if the a/c is on, it idles perfectly at about 800 rpm. The car runs beautifully off idle and has full power with no warning/error lights. It's a bone stock 2.4 with 70,000 miles. Any ideas?

Thanks,
Donny