front end vibration

On a 97 talon,5 speed,non-turbo.What can cause vibrations in the front end?It has new ball joints,driveaxles,tires have been rotated,not to mention balanced 2 times.It will start to vibrate and you can let off the gas and it will stop.This will start at around 55mph and up.This is a friends car by the way.I have no idea what else could be doing this.Thanks for any help....Nathan