2003 BMW X5 trsmision slip

Transmission problem
2003 BMW X5 V8 Four Wheel Drive Automatic 125 km miles

dear sir
i was wodering about my x5 transmision it was working normally one day i reach my work park the car and go for 2 hours i recognize after i come back that the transmision is slipping much after i run for few kilometers the gearbox engage again with normal performance.
the main problem that im having the same problem every time i leave the car especially in the morning time i have to run for few kilometers with slipping gear
action taken: change the oil and the filter of the

trns.
still the same
change the oil pump of the gear
same
changing the clutch and the drums again the problem still there the mechanic said that the gear box inside after he open still in good conition im afraid to change the gear but the problem will be some sensors or sth simple any advice
your support appriciated...