John Deere 550

Hi, my dads boss just gave us an old john deere 550. We didn't get any books with it or anything so we aren't sure what year it is or anything. It also has a problem with pushing, it will push for about 20 to 30 minutes and then doesn't like to push any more. It will back up slowy but wont push or hardly drive forward. We are new to dozer so we aren't sure what the problem could be. Any help would be great! Thanks