high tick over 3.0rpm?

does anyone know what fault it is,have a1999 4.0 r.rover,started it other day an tick over now between 2.5 -3.0!the more i revved it the higher the revs went.have checked throttle cable,its on stop,could it be idle valve,i unplugged it an made no change to revs. put in drive an it drops tick over to 1.0,take out of drive an it goes back to racing!! any ideas please??