Ok this sounds very much like a TPS (Throttle Position Sensor) issue, short of asking you to join the XT Supporter in order to enable you to have the one to one assistance from our mechanical expert Kev, I can only suggest that you ask your dealer to remove, clean and regrease the TPS then fit it back at the correct angle, thus leaving a small open gap for the idle speed to occurr, if there isn't a sufficient gap on closed throttle you can't adjust the idle speed by the throttle body alone, correctly. So try your dealer first and if they are total morons, as most of them are, then come back, join the XT Supporters and Kev will help you to sort it out - guaranteed!

Good luck