i dont know what causes it but on the up trim you might double check your balance it sounds nose heavy . also you can check the ailerons to see that they are in line with the flat bottom of the wing if it isnt it creates a flap effect and requires up trim and also makes it very pitch sensitve which would probably exaggerate the rudder problem i described earlier . take a straight edge and with trim centered place it on the bottom of the wing from le to te there should be no gap if there is adjust it out.