What are the operational differences between a battery and a plug-in voltage supply?
I have determined the cause of the output pulses to be insufficient base current to the Darlington transistor. The reason for the difference in voltage supply and battery seems to be how they act under a load. The voltage supply will maintain a constant 12 vdc no matter the current, while a battery under load will give off ~11.85 vdc. Decreasing the 10k resistor going into the base of the Darlington transistor solved the issue.