It sure sounds like all the symptoms of the classic rearwards CG. But you might check for free play or binding that leads to lost movement in the elevator. I had a glider with a T tail driven by an S bent cable that had some lost movement due to play and stiction and it was a horrible model to find the neutral trim on. I learned to always enter the slow cruise from up elevator and the high speed mode from down elevator. The difference in hte return to the two neutral positions let me do this. Took some head scratching to find it though.