i have something similar but it doesnt happen all the time
Mine is basically like the automatic choke is on but as soon as you drive off its fine and will drop all the way down the revs while in gear still driving, but as soon as you pull the clutch in and free wheel a bit it revs away, but i can get the revs to drop by either flicking it into neutral where the revs drop back to normal tick over or slip the clutch a bit with the brakes on and it drops the revs. Mine only happens every now and then, doesnt matter if the engines fully warm, cold, high speed run, low speed etc
Asked Kev a while ago about it but he's never heard of it.
