uncontrolled acceleration from stop

1998 taurus 24vdohv s.wagon(58k miles)orig owner begun to accelerate without using gas peddle. with both feet on break, continued to run forward for about 100 feet then stopped. Ford dealer repair shop keep for 3 days and could find nothing. Anyone with similar exp advise what it took to fix their car. problem began at 58k miles and has happened three times. each time acceleration seems to get stronger. thanks