You should take the vehicle to the tire store where you purchased the front tires. Your description sounds a lot like either a slipped belt or a bent wheel.
I'd bet on the slipped belt. Radial tires have a very flexible sidewall (radial ply) and a less flexible and tougher tread ply, called the belt. If the material between the belt and the radial plies separates, it will typically slip to one side and only in limited areas of the tire.
One way to check for yourself, jack up the front end and spin the tires. Watch for an area that suddenly goes off to the side. If you see that, it's a slipped belt, and if you have a slipped belt, don't take the vehicle on the freeway and definitely get the tire replaced as soon as possible. They're very dangerous.
Bad tire, wheel, bearing, steering component,..............
== == Bad Tire or Tires.
Your power steering pump and or steering box may be about to give out on you.
Loose/worn steering components. Especially the steering stabilizer shock and the trac bar.
have front suspension checked
Tire out of balance, bent wheel, drive shaft bent, loose or worn steering parts.
You probably need to get your tires balanced. Or replaced. If the bias goes (steel belt breaks or shifts), the car will waddle at low speeds, and the steering wheel will shake at high speeds.
You rotors are warped and will need replaced/resurfaced.
Any number of things can cause a steering wheel to shake back and forth. It could be a bad wheel bearing indicating a wheel is about to fall off. It could be a bad tire or bushing. Without checking it out personally, I can't tell.
Wheels are out of balance
A certain speeds, your steering wheel starts to shake.
sounds like warped or worn rotors to me