OK, I think I got to the bottom of this. I just disconnected all my linkages to do the repairs, and found that the elevator didn't hinge evenly. The tailplane had bowed slightly downwards towards its ends so there was quite a bit of resistance to the elevator movement, but when I pressed it hard enough, it snapped to full movement like a spring. The foam-board I used is very stiff, not at all like the Flitetest board. I did my best with the heat gun to straighten it, so now I have smooth and even movement. I'll try again on Friday when the wind dies down. I should have plenty of time to practice being a right tosser by then.