Lights that flicker may be caused by something as simple as the bulb being loose. Merely tightening the bulb a little or replacing the bulb may solve the problem. Should this not solve the issues there are several other possibilities to explore.
Old switches and plugs wear out
Switches and plugs wear out over time. The more they are used the quicker they wear out. A wore out switch or plug may be the reason for the flickering. Replacing the plug or switch may easily correct the issue. Even tightening the wire connections at the switch may resolve the issue.
The most common causes are loose bulbs, the switches or outlets, or the fixture. Occasionally there may be other causes, such as the wiring or the power lines having a loose connection or corroded connection, in the area where they are connected to the house.
When the Air Conditioner or other appliance that draws a lot of power, are first turned on there may be a dimming or flickering of the lights; caused by a temporary surge in power demand. Also, the voltage may fluctuate occasionally due to the electric companies supply being affected.
Who to consult? An electrician.
Home owners often replace defective lights or bulbs and worn out outlets. Electricians often replace defective outlet, lights, switches and trace down causes for voltage drops.