Many complaints have been reported regarding Volvo S80 transmission shifting issues. Long shift times between gear shifts, hard shifting, hard downshifting and a loss of transmission operation all together to name a few.
If the issue is minor, a transmission software update may address this issue so if available, the software should be updated before any repairs are made. After a software update or repair, the shift adaptation needs to be reset. A good quality Volvo repair shop will know how to perform this task.
There are several technical service bulletins (TSB's) available from Volvo that address these shifting issues and they should be consulted by the repair shop during the automatic transmission diagnostic process.
Regular servicing of the transmission fluid can help with preventing transmission failure, but not in all cases. Follow the suggested fluid replacement interval recommended by Volvo. You will find this in your owners manual, or find it here: http://bit.ly/Volvo_Owners_Manuals