External watchdog reset for AVR required
I normally prefer to determine the underlying cause of the periodic failures, especially if it's purely software related, but external power management and watchdog solutions are available. They can help remove the possibility of some software conditions causing the watchdog timer to become disabled or being 'kicked' falsely.

The first manufacturer that comes to my mind is Maxim and they have a Comparison of Internal and External Watchdog Timers application note that is worth reading. The Datasheet that covers the MAX6746-MAX6753 shows some of their parts available. The reset and watchdog delays for those parts may be changed by an external RC network.

If you have software problems you might want to consider the MAX6752 / MAX6753 that have a windowed timer. That means the watchdog will reset if the watchdog is trigerred too fast as well as too slow. If those parts don't seem suitable a search for "watchdog" on Digikey shows many results under the "PMIC - Supervisors" category that may be suitable.