do you have adjustable sway bars in it? If so soften the rear a touch.
If not, then yes, a softer rear spring will help.
You can also play with more rear toe in.
As far as the adjustable damper goes; you are adjusting both bump and rebound at the same time so you need to find a happy medium. I generally would go somewhere around 10-11 front and rear and pretty much leave it or go up, but not down. I tend to think if you go too soft on the damper thinking that it will soften that end of the car then you aren't controlling the spring properly, particularly higher spring rates, which can lead very similar issues to what you are describing...