Vibration problem! Help!

[panda78]

Hello,
i was driving on the highway this morning , after about 80 miles between 65 and 75 mph speed , my steering wheel starts to vibrate a lot !
I tought i had a flat tire , so I stopped on the highwat , but all the tires were fine!
So i restarted again , everything good for a couple of miles , than the vibration come back again , but they went away after 1 minutes.
After that the car stayed parked for about 8hrs , then drove it again for 30 minutes with no problems.

Any suggestions?

Thanks!