Hi there i have an a4 1995 1.8. When the car is cold and idling it runs fine at 1200 rpm. When it warms up the rev counter goes from about 1200 rpm down to about 600 rpm and up and down for 20 /30 secounds and then levels out at 850 rpm. Not sure what to do so any help will be gratefully appreciated. Many thanks from ade

 

You should post this in the A4 section, but it sounds like an Idle stabilizer valve.