I've got a board with a PTN78060 that is regulating 28V down to 5.7V. The board was designed about 18 months ago and we've made >20 boards that have been working fine. Out of the most recent batch of boards we've recently had ~4 boards in which the regulator stops regulating and the 28V appears on the output pin. I've got a 330uf cap on the output pin and a 10uf ceramic cap on the input. What can cause the regulator to fail in this manner?