AWD Transmission: XC90 help needed

[Dunc56]

I bought my XC90 D5 AWD in a private sale - sold as seen! I had only driven it about 10 miles and ‘Transmission service urgent’ came on. I have had it looked at by an independent Volvo specialist, who replaced to transmission fluid. However, the problem keeps coming back. When the warning light comes on, I can not select manual gears and the display just shows a - there is limited gear change. If I pull over and stop the car, then restart the problem disappears for a time. Anything from a few miles to several trips - up to about 75 miles is the best.
The OBD2 codes are:
P0735 (ECU 18) Gear 5 incorrect ratio
P0501 (ECU 18) Vehicle speed sensor (VSS) range/performance problem
P0700 (ECU 10) Transmission control system MIL request circuit malfunction
Does anyone have any thoughts on this problem, it seems to me - a novice at such things, that if just by stopping and restarting the problem goes away, it can’t be that serious, but the garage seemed to think that a replacement gearbox was needed.