Video was 128 averages. Tried it again, this time 8192 averages. Took what felt like about 10 minutes but basically the same result. It's almost as if during the accumulating of traces the dither frequency between an upper and lower pixel is relatively low, giving just a single non-returning step during a sloping part of the waveform. When the average count has finished the dithering rate seems to increase greatly, giving the appearance of a blurry trace where it gradually progresses in a sort of PWM fashion from one horizontal row of pixels to the next one down. It might be technically correct but I don't like it!