2007 Audi A4 Quattro 2.0, 80,000 miles ....With the engine cold I get a variation of rpm's at cruising speeds, 1800 to 2400 rpm's, usually around 50 to 150 rpm's. There is no engine light on, sometimes the car will run fine...Kind baffling , Does anyone have any idea's in what I should check or replace...Thanks

 

air in the coolant lines can't cause idle fluctuations. have you done a full coolant change yet?