We need to know more about your application and how is it used. It seems to me that the latch-up effect could be the source of your problem. If so, you must protect your IO pins against excessive voltages. If the voltage rises above VCC or below GND and the current spike is high enough parasitic PNPN or NPNP is triggered (like firing the SCR) thus causing large currents to flow and MCU stop operating. If VCC is disconnected, the system returns into normal state again and when applying +5V begins working as normal till the next high IO pin pulse that can cause the next LATCHUP.