Wheel balance problem

I Recently replaced the tires on my 1998 Maxima SE with new tires (215 55 R16).

At about 70 mph and beyond, the car feels like it has a major wheel balance problem that has developed just since the tires were replaced.

After 3 balances the problem is only getting worse. The car used to ride great with the original tires but now rides very badly, especially at highway speeds.

Do you have any ideas what may be causing this?