Most such problems which happen after a specified time can be due to a stack overflow. You can check on that aspect. Be sure as to where you intialize youir stack.

Another strong reason for such behaviour is uncontrolled EMI that sets in when some highly inductive load like a motor / solenoid etc are switched ON. Total failures are "easy" to diagnose. Randome failures can make you go bald and still remain unresolved. First try to establish a "method in the madness" in other words find the causative factor thattriggers a crash. That could be an eye opener.

Time to check your grounding practices also.Check this forum for topics of EMI or RF interference.

Raghu