I have a very straight forward arduino/Xbee series 2 pro setup. Everything works. Ever 5 seconds the sending arduino transmits a string to the receiving arduino. It works perfectly. If I change the delay to more than 5 seconds on the transmitting arduino, it seems that the xbee on the device loses its connection and resets.
Are the radios sleeping and then waking up after 5 seconds? If so, check the SP and SN of the parent device. A parent device will kick a child device from its network table if it does not receive any communication within a set amount of time.