Up and down RPM when cold

Have you checked:

- for vacuum leaks
- throttle position switch
- temperature sensor
- cleaned the base idles screw & passage in throttle body
- cleaned & checked idle stabilizer Milli amp current
- is ignition timing correct
- fuel pressure
- are both fuel pumps working correctly
- is the fuel filter dirty
- how's the air filter

I could list more things. Point is, you need to tell us what you already checked.

Also, what engine and fuel injection system is in your cabrio, since different things apply to different engines.

Thomas