My 300-ce rocks from left to right

Hi, i really have a serious problem,
since a month my car when driving on the hi-way rocks from left to right, it feels like my tires a loose, but it isn't

I went to a garage ant they dit a full check and replaced some rubbers in the back, but still there is no change.
This causes really dangerous situations on the hi-way because i really have to work hard to keep my car on a straight line.

I have replaced also my shock and spring but still no change,

I hope that there is someone who knowes what and where to look for this problem.