Why would Transmission hang up in one gear and display goes from "D" to "-". on 2001 Volvo V70 XC

2001 V70xc AWD Cross Country Transmission will hang up in a high gear when car gets hot. The gear display will go from "D" to a "-". Car will still drive but when stopping, it is like starting off in 4th gear. Turning the car off and restarting it will reset it temporarily but will soon hang up again. "Transmission Service" message is on all the time as well as "Engine Service" message. Will replacing the transmission computer solve this issue?

Asked by for the 2001 Volvo V70 XC
Take it to a transmission specialist for propper diagnostic