I think I figured out the source of the hang, so I spent a few hours wiring the last two motion detectors into the SensorMonitor. All four are now operational.
The hang has not occurred for over a week – it’s never taken more than 3 days to hang in the past. The only thing that has changed is the LED is left off, except when the udpSend call occurs (allowing me to verify the hang occurs inside of udpSend).
I came to the possible conclusion the problem is the LED being turned on and off. It uses pin 8. I started researching the ethernet card to see exactly what pins it needs. While my card uses pins 10-13, some use pin 8. The driver is configured to use pin 8. I discovered that there are 2 places in the driver where the Device Select pin is encoded. I changed one from 8 to 10, but didn’t know about the other. I suspect that is the source of my problems and I’m headed down that node of troubleshooting.
This post was how I discovered the problem:
http://arduinodiy.wordpress.com/2013/04/07/connect-an-enc28j60-ethernet-module-to-an-arduino/
Big Dan the Blogging Man 