Temp effecting an automatic trans is normally the sign of the fluid being low or a restriction. The front pump can also cause this.
Though normally on a bmw I automatically think low fluid if it does that cold and not warm. Also sharp turns can cause it to do that when the fluid is low.
The fluid is thicker when cold and takes longer to return to the pan. Thus when hot there might be enough fluid to pickup and pump but when cold its not coming back to the pan fast enough to draw back in. Its really has to be bad low to do this on a trans though. Motors can experience the same problem but it doesnt have to be near as low on oil to get the same effect as the oil in a motor has a good bit to travel via gravity to return to the pan for the sump to pick it up again.
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