Engine idle question for you

Hello there, I'm curious if any of you have had an issue with your engine periodically idling too high? I have an '06 X1, RTP 310 hp, and every now and then, when I bring the throttle to neutral, the RPM's stay at 1200 or so. It seems to do it when the motor is hot. I then push the throttle release button, and quickly hit the throttle and the RPM's drop back down to 600 or so. It almost seems as if something is sticking. Have any of you had a similar issue? The dealer could not duplicate the symptoms, and therefore, could not fix the issue. It reminds me of a carburated motor, idling high when warming up, and you hit the accelerator to drop the RPM's down.

Any thoughts?

Thanks for your help.