First question : what tyre size are you running front and rear. Generally, 19 inchers with low rubber height (serie 35 or 30) will make the car much more sensitive to road surface imperfections. You will then have to be more "awake" during main road cruising.
Another point : 2 years ago, I remarked one day that my Supe was very unstable, which was very noticable on rainy roads. After a complete check, I found that 1 rear spring was partially broken (not completely) and caused oversteer on small bumps even when driving at low speed (no exterior sign was seen ... like the car being lower on that part). Afer having replaced the spring, everything was fine again.