My car shakes at higher speeds 60-70 but not all the time. Sometimes it is smooth as can be. It can do both on the same drive. Shake, slow down, speed back up and no shake. Ride smooth, change speeds, speed back up and shaking. It also shakes while braking at higher speeds but also not always. I would assume rotors but I just replaced them 2 weeks ago. I also just rotated and balanced my tires, checked for any loose suspension parts, and checked for bad bearings. This one is confusing me. Not sure what can cause this only sometimes. Help!