I have the same problem with my 9C. It is to do with the processing of each channel, which is done in sequence i.e. 1, 2, 3, 4.....8 etc. and there is a slight delay between each channel. So when mixing channel 2 to 8 there is a delay between the two servos. Using ailevators helped a bit but not much as at the end of the day it is still mix between 2 channels.