I agree on FLTDI's advice, but I'm worried that's not your culprit. If rewiring the switch doesn't stop the flicker, keep checking other outlets and lights on that circuit until you find it. I had a light recently that flickered whenever a lamp on the same circuit was turned on. I dragged my feet on investigating, and a couple of days later, it went completely out. When I took down the light and looked at the wires, they had burned up. I was lucky the heat/fire stayed in the box.