First, make sure that the center of the models is in the center all 3 axes. For this, of which works best with the MED in 6.4, select the entire model, take the vertex at the lowest point and move it up to exactly on the Z axis. Take the topmost vertex and divide the position by 2 and set the position that way. Repeat for the other axes and this will exactly center your model. Doing this should help.