After completing the parking checklist and connecting the external power, I switch the APU BLEED to OFF then I wait 5 seconds and I switch the APU MASTER SWITCH to OFF but, in that moment, I get a MASTER CAUTION of APU AUTO SHUTDOWN and the FAULT LIGHT comes on on the APU MASTER SWITCH pushbutton. It would have been cool if it happened once every 20 flights...but not in almost all of them!!  Five flights completed and 3 of them experiencing this FAULT. Quite frustrating, isn't it? An