Help needed

Having a few problems with my 156 with 2.0 l engine&155 BHP. When car is in neutral gear revs are climbing from normal 850 up to 2000 where they hold for couple of seconds and after that revs are coming back to normal.While this is taking place I have noticed that temperature is climbing slowly and is between 70&90. All this is more intensed when I switch on airconditioning on. I have checked my car on diagnostics and final result was that everything is normal and yet revs are doing the same. I have changed part for neutral gear revs cleaned the entire injection unit and still nothing. Has anybody else had problems and if so what was the solution.
Many thanks
Tumbas

Id have a look at ur air flow system,filter,flow meter,pipes etc