Check the carrier plate
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2006 Volvo S80, 2005 Volvo S80, 2004 Volvo S80, 2003 Volvo S80, and 2002 Volvo S80. However, it is still accessible from the selected car via links, so may be relevant.
Check that the carrier plate is not damaged or cracked.
To replace the carrier plate see Replacing the carrier plate for S80 gasoline.
Check that mating surfaces of the engine are clean. Check that the guide bushings for the transmission are intact and in position.