There are two solutions left IMO:
A) the model has to be corrected before it gets imported to MED.
B) in MED you erase the position values of the bone that is parent to all other bones. Before you do so, you have to investigate the moves of this bone. Its moves are most probably an 'up and down' in the walk animation. After erasing its position values, the model should have the origin in its middle in each frame. Then you have to adjust its position in each frame.