Are you providing enough capacitors for each device for decoupling? Does the power supply have adequate "reserve" power to get over peak demands? Are you decoupling all signal lines that leave the board? Are all unused CMOS device inputs tied to ground? Is there a RETI instruction at all unused interrupt vectors? Are you absolutely sure the Stack isn't sending your program into the Enchanted Forest? Have you considered adding an external watchdog/reset monitor?

I'm sure this list will grow as others respond.

Hal