Is the micro getting abnormally warm or even hot, when it hangs? How do you exactly hard reset the micro? Is it actually a power-down followed by a power-up? Or is only a reset needed, without a power-down and power-up?

You can get the micro locked-up when glitches or ESD hit the port lines or Vcc. In such a case, it cannot be simply reseted by a watchdog, but only by a complete power-down and power-up.

Another idea is, that your watchdog isn't working properly, because you haven't configured it correctly. Have you connected the /EW pin properly?

In my projects I often use an external watchdog, like in the one in the MAX1232, as shown in this example circuit:



Kai Klaas