At idle the engine revs up and down

Engine lite on? would ck to make sure there is plenty of coolant in system. low coolant may cause idle hunting. also could have a vacuum leak or air intake restriction. ck air filter

I have a 99 C70 convertible with similar Idle hunting problem. when cold idles fine but when warm and in park it hunts . In drive or rev it idles fine .

I'd check the ETM (Engine Throttle Management) module. It is up under the fuel rail on the intake side of the air inlet. It gets deposits on the butterfly valve and it lets air in when it's not supposed to. Confuses the MAF and ECM. You can remove it and clean it up and reinstall. Also check your vacuum hoses. Over time they get brittle with age and heat. Cracks in them can leak to the ECM trying to adjust fuel/air mixture.

Check to see if the intake hose from the filter has no holes in it. If there is a split in it after the Mass airflow sensor, it can do that. The computer gets super confused, and keeps trying to change the air fuel mixture.