fast idle when warm

I have an 87 911 that fails to return to idle after the car is warmed up. At first I thought it was a linkage problem, but then I notice it never does it until after it reaches normal operating temperature. If I blip the throttle, the idle drops to normal. It is rather disconcerting in traffic though and will act like the cruise control is on at highway speeds.
My question is , what could it be? I note there is a sensor that apparrently signals a full release of the throttle. There is also some sort of switch on the side of the throttle body. Anyone had this problem before?