The car floats on the road at higher speed.

Hello, first, I would perform a thorough check of the steering and suspension components. Focus on any spring or shock absorber that is either broken or conversely binding. Next, I would have a thorough wheel alignment inspection performed, including all angles including SAI -- steering axis inclination. Ensure the rear wheels are aligned properly to the front wheels, and are within specification. Inspect the tires to ensure they have not aged over 5 years, and ensure tire air pressure is set to the proper level as indicated on the placard on the driver's door jamb. These are good starts, and hopefully will reveal the concern to you upon inspection.