Have you taken any of the pumps apart to see if any small foreign objects are getting in there to lock it up?
I've had one pump lock up on me last year, and it turned out to be one of the very tiny metal spacers that ford puts in the FACTORY valve cover gaskets that finally made its way down into the pump. I think it was lodged somewhere in the head, and then when I pulled the heads off to change the gaskets, it loosened it up and decided to head down into the pump
Are you using the same brand of pump everytime? If so then it could be a manufactures defect