high idle after long drive

hi guys

got the 1.9 gti engineering lump in my car, with tsr cam, 4 branch etc.

when driving long distances (bout 20 miles) or after a good rally session the revs just push themselves right up to 2-2.5k… i have left the car to cool right down then turned it back on in say an hour and its still that, i have to turn the idle timing down for it to go back to 1.4k

does anyone know why this would be doing this?

Had this prob on my brothers GTi, the idle screw in the back of the throttle body came loose and kept working its own way out, used a bit of pTFE tape to hold it in.
Haven't had this problem since.  

so would changing the throttle body (which im gonna do anyway) to a mk2 16v bigger one have a screw on the back too?

Will have a look when I get home from work, as I have both sat at home in my garage, will check to see if its a mod that will work too.
Have only heard of people changing Mk1 throttle bodies for the Audi 2.2 5 cylinder ones.