Of all the symptoms that an electrical system can have, groups of lights that flicker are (in my opinion) the scariest. I suspect that you are getting some arcing somewhere. Arcing occurs when wire connections are close but not fully touching. The electric current jumps the gap, like lightning, and creates some heat. This heat could be minor, or it could be a serious problem, depending on how much current is jumping across the gap, and how wide the gap is.

This is not the same as a short circuit, although both can give off sparks. A short circuit happens when the hot wire touches the neutral or ground wire, and normally the circuit breaker or fuse will trip, shutting off the power.

Arcing can occur for hours, days, weeks, even years, and it will never trip a conventional circuit breaker. And arcing can cause enough heat buildup to start a fire. When in doubt call a licensed electrician.