I would try pulling the power off and see if the thing will glide stably. If not, move the CG ahead, and then try reduced elevator throw. Almost all the RC models that I have encountered need larger vertical tails. Remember the words of Frank Courtney, WW2 era experimental test pilot: "I have never flown an airplane with enough vertical tail area." A quick and easy way to test for lack of enough vertical tail is to tape a cardboard extension to the rudder, decreasing rudder travel in proportion to the added area, if you already had enough rudder authority, although I have never flown an airplane with what I regarded as excess rudder authority - the more, the better, in my book.