1990 Volvo 740 turbo rough idle problem?

You don't mention the top suspects - the throttle body and Idle Air Control valve. Volvo cars of that vintage have no PCV valve, but have an oil/air separator and some hoses. In turbo models the hose runs right to the turbo inlet and introduces prodigious amounts of oil mist into the intake. I'm sure you've noticed the oil coating.

Anyway, that oil leaves deposits in the throttle body and IAC valve. The best way to clean them is to remove the throttle body and the IAC valve mounted below it, and spray about half a can of carburetor cleaner through them. The vane inside the IAC valve should move freely when you are done, clacking when you rotate the valve sharply in your hand. Be sure to get lots of spray through the idle air passage that is near the throttle plate when it is closed and to remove the ridge where the plate sits when closed.

I did the cleaning every year or so, when the engine started dying when started cold.