High idle at speed, not when stationary
 

I recently swapped my engine (it's a 996 Carrera), and got a strange problem. It idles high, at 1500 rpm whenever I clutch (with accompanying poor mileage). It will continue to stay at 1500 rpm, making me look like an idiot around town, until the car has been standing still for 10-20 seconds. It will then idle down to approx 900 rpm like it should. I swapped over the throttle-body from the old engine and it worked perfectly before.

I checked the throttle valve and cleaned it out, but it did close properly already. Tried to clean the ICV as best I could, but without liquids. I did notice a clicking-sound from it that I'm not sure I've heard before? It may be related?

Any ideas?