With the right combination of tires and racing surface, having too stiff of a rear setup and too soft in the front can take steering away. This seems to be more prevalent in really flexible chassis touring cars as well.
I'm going to guess that you're full saucing your front tires as well, and your prep is good, car is properly built and not tweaked, from your stated experience with nitro
Does the car track and steer well during 50% speed laps but only start to push badly at race speed? If so, to me this is a classic symptom of the car dragging the edge of the chassis in corners. Depending on your racing surface you may not even have a rub mark or goo to indicate it. Applying a small amount of white-out on the underside edge is a good way to tell, and wont stain or other ugliness.
My order of attack: find out if the car is dragging, if it is, harder front sway bar until it doesn't drag. Then you can tune with the weight of the car on the tires.