Increasing the tire pressure makes the contact patch smaller, but I would do it anyway to tighten things up. What is the recommended tire pressure for running at 100 mph? That might be something to try.
Have you noticed a slight loss in performance since the bigger tires are a little heavier and the circumference of your tires is larger, which if effect, makes for taller gearing? It takes fewer engine RPM to move the car the same distance because the tires are bigger. Have you noticed a slight increase in gas mileage?