My 2005 sprintshift goes from Auto to 1st intermitent ??? Y ? ?

I have a 2005 Sprinter with Sprintshift, it drives lovely then sometimes when I stop & pull away it goes into 1st gear ?? So I drive it using gears, then when I stop put it back into auto & it can be ok for a while then does it again, a day or weeks can go bye before it happens again. Anybody have the same fault or know anything then please email or call me please Mark 07789 646 567