Hi matey, as stated get your wheels balanced, but just as importantly get them aligined properly. When i first got my car i had almost exactly the same issues.
Felt fine until i got upto motorway speeds when the backend felt like it had a mind of its own!!! Very scarey!!!!
I took it to a garage near me that has the proper laser alignment tool. Turns out the rear wheels where off my a few degrees, way over acceptable levels.
The car nows feels perfect to drive, made such a massive difference. Also got the dealer to refund the work as it wasn't sold to me as it should have been.
If you haven't had the alignment done yet that would be my first thing too do.
Hope it helps.
Regards, Lionel.