If you check the control system and there's no slop in it then what you are experiencing is not related to the elevator but to the flat airfoils.

Flat plate airfoils create a separation bubble just behind the leading edge on the low pressure side of the wing. The overall airflow passes over that stagnant bubble (think of it as a back eddy in a stream) and then sticks back down to the surface behind the bubble. So the bubble is actually becoming a flexible part of the airfoil shape. As the angle of attack changes the bubble changes too. But at higher speeds and lower angles of attack this bubble becomes very small and tends to bounce from the upper to the lower side of the wing. This really messes things up as you can imagine and the model will seem like it's hunting up and down as the bubble snaps from one side to the other in flight.

The pizza box flyer guys notice this all the time if they try to fly too fast. It can also be exagerated by the flexibility of the wing. And the PBF's and flat foamies DO have fairly flexible wings.

If you slow down and fly so that the wing has a more or less easily noticable angle of attack you'll probably find it's fine. Only if you go faster does it show up.

PS: I seem to remember reading that the area ahead of the hinge line should not be larger than 15 to 25% of the area behind the hinge line. If it gets to be more than that you will have problems with the balance area trying to fly the surface.