why does my transmission shift to neutral while driving and won't shift again? on 1999 Mercedes-Benz ML320

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i was driving when it just went on free willing.i pulled over.restarted the engine ,put on drive but after 50 meters it did the same thing.i have to restart engine ,drive 50 meters and the cycle repeats.
(1) Answer
(1) Comments
check the fluid level, beyond that it is an internal failure from either front pump or seals failure

i flushed the tranny today and filled it up with the specified fluid.5 qts it was shifting smoothly on D,but won't shift on the manual gears.added another qt,now i'm back to the same problem,it happens when the engine has warmed up to 85 degrees check engine is not on.can i still get a code with a reader?thanks for the reply.