One problem related to transmission gear slipping has been reported for the 1999 Volvo S80. The most recently reported issues are listed below.
Replace main computer after computer registered "urgent, brake failure" code and Volvo determined it was a faulty code. Three months later, I had transmission slippage while up shifting. . . Getting extremely bad. Had transmission rebuilt at a reputable transmission shop and he told me the transmission (at 80,000m) failed because it was never serviced properly. I have all service receipts. . . All done at Volvo dealerships through 50k warranty. Transmission shop said, my gm transmission, unlike earlier Volvo transmission needs more servicing than a Volvo transmission. Transmission shop said that my "transmission filter had never been changed!!!!" contacted Volvo consumer affairs and they basically laughed at me. I am thinking about filing a complaint in small claims court for 2k the cost of my rebuilt transmission. President of cars suggested that was a viable option.