In later year 90 series cars (after 1996), the mass air flow meter uses a hot film sensor that can become dirty. To clean this sensor, remove it from the air box outlet and follow the instructions in the photos below.
1. Unscrew with T20 Security Torx
2. Dip this end in Simple Green 1:1 solution
3. Clean gently with q-tips
4. Shake off and dip into 99% alchohol
5. Dip mesh end in Simple Green and then Alcohol
6. Screw clean sensor back into tube
Now reinstall it back into the car.
Volvo Maintenance FAQ for 7xx/9xx/90 Cars