Remove some elevator. Too much elevator will cause it to rock. Anything more than 45 degrees is wasted. This has held true in several planes I have had. First the H9 Edge. Then the Phoenix models Funstar. Then the Funtana 40 and now the H9 Sukhoi. All had a elevator and harrier problems until I removed a little elevator. Just put it into a harrier and when the wings start rocking, just remove a little. Of course you could move the CG back and that will probably help since you said it did fine with an extremely rearward CG.