I think this is a combination of a few things, how hard do you have the suspension set too (how many clicks etc)? and the spring rates, those spring rates are very aggressive.
Area's version of these (which they use on the civic cup cars are recommended to run 8kg front and 16kg rear), you have 20kg rear which is very harsh, this issue maybe (depending on how hard you have the suspension set being a case of the springs being too hard and the dampers set too soft to control them properly)
I would say play around with the dampening a bit (if it's soft then see how it acts when stiffened a little more and vice versa, it will also be different for each track/surface you are on.
More seriously though do you have the harness tied to the harness bar? if so that's dangerous, a harness bar is only to be used as a harness guide to allow the harness to be within the required degree of angle and not to be a support to hold a harness bar, if you have an accident the bar will bend and the harness slacken ending in a bad way for you. In my build thread I show how I used to have a harness bar in my car and where to mount the harness as recommended by Takata etc when not running a cage.