Have a full flying stab with a futuba S9405 servo (100oz/torque), when I apply and hold full up elevator I can take 1 finger on both sides of the stab and push it down, then I let go it 'pops' back up (still holding the stick on the radio)

Did the same thing on 2 different battery packs that were fully charged.

Weak coreless servo?

You may have used a longer than needed servo arm and reduced the throw with the travel adjust on the TX. This will reduce the torque.

I would try a shorter arm on the servo and extend the horn on the surface if needed and try to avoid using large amounts of ATV to reduce the throws. If you find you need to then use the shorter arm on the servo or longer on the surface.

If I was setting up a all flying tailplane then I would look to get about 110 each way on the ATV which would increase the torque to the surface.

Jason