Ya, as it says, if you can hit 5800 it's under pitched and you'll lose top speed. Back it down until you hit 5600 and you'll get your missing speed back. As for your stability issues it's just an effect of having such a thin elevator. Thin airfoils are very angle sensitive and will slowly move upwards and downwards as vortexes shed off the surfaces in an alternating fashion. They do not have laminar airflow and are prone to vortexes forming along the leading edge during any change in AoA.