2001 Max Trans Slippage? ...

My 2001 Max trans has began to slip intermittently, but my dealer tech says he can find nothing mechanically wrong with it ... I had the fluid changed, it was not burned or dirty, and neither were there any fragments in it ... If I turn the car off, then restart, the car changes gears as it's always done ... Could there be a computer and/or sensor problem (but I've gotten no codes via service engine soon light)? ... Thanks