Skip,
Is your situation that lights are turned off randomly but only if the ELK is connected to the PLC? Or if the Serial PLC is plugged in but the ELK not hooked up to it?
The only time I see lights turn off like that is because I have a rule that states if the system is armed away and its dark out to turn certian lights on for X amount of time depending on the day etc. Then if we come home late and disarm and the lights already on it will turn off after the X amount of time from when it came on even though we are home and disarmed at that point.