my car shakes and I don't know why?

What is your question?

when I am driving on the highway and I set my cruise control to about 78mph, my car then starts to shake. For instance if I look out my rear view mirror, it is shaking so much that I cant focus on words very well behind me. if im driving at night and reading a sign in front of me my lights are shaky. I just got all new tires with an alignment and ive brought it in twice already and they've checked that my lug nuts are tight and my tires are balanced and that my alignment is in fact correct. the problem doesn't occur on a daily basis because I don't drive over 60mph. if I have my cruise control set lower than 70mph the shaking doesn't occur. its very unusual and no one can seem to figure it out. any ideas?