Volvo: Transmission 2004 xc90 t6 awd, newer cars, service transmission

Question
My xc90's transmission started slipping and the 'SERVICE TRANSMISSION' message came up with no check engine light. I took it to the dealer to get a flush and fill. He also changed the filter. It is a week or so later and it has started slipping again. When I am stopped at a red light, the light then turns green, I press the accelerator, the rpm accelerates but not the vehicle right away. Then as I am driving it slowly picks up speed.

Answer
i don't know how old the car is but shifting in newer cars is controlled by a seperate computer each gear has it's own solenoid, so it can get more complicated than a flush and fill, i have no idea what it could be, i would be zeroing in on the computer