My name is Vishal and I work for the Social Media and Community Team at Dell.
According the scenario that you explained above I have found out that there is an issue with the connecting cable from the LCD and Motherboard. The cable is faulty, and when you connect the external monitor, the electricity balances and the Display on the LCD doesn't flicker. Hence you should think of replacing the Cable which connects the Motherboard and the LCD.
In case of any further help, please reply.