Optical isolator is worth looking at, never used one before. 
  
I can reconfirgure the 232 to not be connected to the texecom, effectively the C4 is a virtual keypad so always checking for data with the texecom and reporting back and forth so i wonder if i bad response causes the crash. 
  
Think i need to spend some time making sure i can create the fault and then try firmware updates first. 
  
Is the CIE a common fault? 
  
The building is quite large and some of the C4 kit is in a second rack on a different mains phase, could this be an issue?