2005 Sebring bounces between 45-55mph

You might suspect a broken engine mount that is letting one side of the engine sag. That can allow the half shaft on that side to contact the inner cv joint housing. It will make a pulsing feeling three times per tire revolution. That can also open up the spring-loaded gasket where the exhaust pipe is bolted to the engine. Have the tires inspected for a broken belt, then have a mechanic test drive the car. They will pick up on clues and hints to help find the cause.