As mentioned, start with maint items:

Clean the throttle body and Idle air control
Check the accordion hose for cracks while you have it off for the Throttle body cleaning
Check and replace the air filter if needed

Since this is likely an LH 2.4 fuel injection car (normal soup can-looking coil) you have two likely suspect if the above does not fix the issue you are likely looking at some other large vac leak, a failing AMM, a failed cat or way down the list a bad O2 sensor (should not significantly effect idle).

This is all assuming that if you have an automatic transmission you are not just dealing with a very low fluid situation that has the pump cavitating.

Do this, also:
https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm

You may have a host of causes. Research this board using the symptoms and perhaps you'll find a solution.

Maybe a minor tune-up.

Else, check the OBD-1 socket 2 (fuel) and socket 6 (ignition) and see if the OBD on your Volvo 240 has any fault codes stored.

cheers,

dud.

That's a practicly new 240! Sounds like a clogged catalytic convict. Blocked fuel filter. Blocked air filter. Extremely dirty throttle body. Hope that gives you a start.