Volvo Repair: volvo 940 rpm surging, 1995 volvo 940, volvo 940

Question
1995 volvo 940 sedan, rpm rises and stays at 2-2.5K while in neutral, drops down somewhat when in drive. Seems worse in cooler conditions. Will also idle higher(2-2.5K) at times in park if you touch the gas pedal. Sometimes when idling high, if you rev engine and take foot off gas, it will return to normal idle of about700-800rpm.  

What could be causing this? Throttle body already cleaned, mechanic having trouble solving this intermitent problem.  All suggestions appreciated.

Answer
if I had to guess, I would guess that the constant idle speed motor is sticking sometimes.next time it sticks try tapping it wiht a screwdriver and see if the idle drops.the idle speed motor is under the intake manifold.it has a connector and 2 hoses attached to it.