I had the same "bouncy" feeling on a brand new set of 18s I just put on my car....this is how I resolved the problem:
I knew it was not my suspension (BAT struts/Eibach) as it less than 10 months old....and my car is not a daily driver. Also, it had just spent an hour on the alignment rack prior to the install of the tires.
It turns out the problem is that the tires and rims needed to be "indexed" or "ride matched."
Tire Kingdom in Miami has a machine that puts a load on the tires under speed and determines the high spot on both the tire and rim. The tech then deflates the tire, turns the tire on the rim to the correct place (as determined by the machine), and puts air back in the tire. This whole process took about 2 hours...what a difference. My car is glass smooth @ 95+....not that I drive that fast.
As an aside, I paid Discount Tire Direct an extra 40 to ride match my wheels/tires before shipping them to me...obviously this did not happen. I called them and told them about the problem....they told me to resolve it at the tire shop and they would foot the bill...they did. I recommend them as they stand behind their product.
