I'm not sure what you mean by flat spot when the car is jacked up.? Is the shaking through the car that's you can feel it through your seat? Or can you feel the shaking through the steering wheel.when there is some shaking does the steering wheel shake left and right quickly? If the cars smooth up to 60 and smooth after 70 but comes on between that's a typical out of balance. Does the shaking go side to side or up and down?also when you brake does one or both front wheels shake and is it side to side?
I ask these questions to try to understand the cause! As any one of these questions you answer helps to pinpoint the problem. So the front wheels shake between 60 and 70 that indicates a balance problem,but when you brake you get a shake , that would indicate a warped disc!. Please answer the questions and see what we can come up with. Regards Fred.