Sounds like your battery may be weak. The Volvo consumes quite a lot of power running the fuel pumps and ecu, along with whatever else may be on such as lights, radio, heater etc. It will sometimes consume more than the alternator can produce at idle speed. If the battery can't maintain enough peak voltage, the problem becomes noticeable. Check the battery voltage at the terminals with the engine off and with it idling, it should remain close to the same.

The voltmeter is not highly accurate; I would check the voltage at the battery to see if it's unacceptably low. Make sure your battery terminals are clean and tight! Alternator belts tight enough? Idle speed high enough?