By how the carbon tube seems to be unsupported minus the ribs, its not surprising that it didn't help the problem. It really needs to be part of the rest of the structure by tie-ing the top/bottom skins together. Using some contest grade balsa to fully sheet the top/bottom of the stab as well as placing some 3/16" balsa sheer webbing ontop/bottom of the CF tube to join the tube to the new skins will likely fix the problem and have minimal significant impact to the weight of the model.