I have a problem that I have not seen discussed on any web postings.  I purchased a 6 quart Professional KitchenAid specifically to kneed bread dough.  I make a variety of yeast breads and many require kneading for 10-12 minutes on speed 2.   The mixer often overheats.  I have discussed this problem with KitchenAid and they have told me that their mixer was not built for sustained operation of 10 minutes or more and that I should follow their directions which call for a total of 6 minutes to mix ingredients and knead.  I have made both white and whole wheat bread following their directions and have found that in all batches of dough, the gluten was under developed.  It may be that I am pushing the mixer to the limit but all of my recipes call for less that the 14 cups (about 59 ounces) stated in the KitchenAid instruction book.