PTH04070 multiple damages

Recently we have about eight PTH04070WAD modules permanently damaged one after another during working in our device where they provide supply voltages for Xilinx FPGA and CPLD, Ethernet PHU, DDR SDRAM and some other minor circuits. The device consists of three PTH04070WAD modules supplied with +5 V from external stabilized power supply unit and provides 1.2, 2.5, and 3.3 V outputs. The modules got damaged one at a time on in couples without any obvious reason for us. Before being damaged they work correctly for a considerably long period of several days. The way by which we use PTH04070WADs  meets the recommendations given in the data sheet with the only exception that we use considerably large output capacitance of 3000 uF in order to provide supply for our circuits during a short period after the external +5 V is switched off for safe termination and storing variables in non-volatile memory. Damaging of the modules results in switching the external supply unit into current-limiting mode or in some cases in disappearing of the output voltage of the module. The output of the external supply unit has transient voltage suppression circuit which prevents possible pikes and over-voltage. Could you tell us possible reasons of such a behavior and means of correcting it.