What you might have is the two rear servos move more than the elevator servo. I think I had the same thing happen when I did that setup. Just adjust the travel adjustments for rear servos so they don't go down as far. Are you doing this in throttle up mode? Did you take the servo horns off then turn on and off the transmitter? Then put the horns back on close to 90 degrees? Then did you use subtrims to make it perfectly 90 degrees to the servo?