Start with a screen diagnostic -- hold D through powerup.  Does the issue show on the test screens?

Flickering monitor issues can be caused by loose connections or faulty cables. Before attempting any other troubleshooting, check that the Dell monitor cable is securely attached to the video input on the back of the computer and to the video output on the back of the monitor.

Its a possible issue with the Display Cable or Display itself ...You can check with any repair store and get the faulty party replaced