Do your lights dim every time something electrical powers on? Often this is due to a poor neutral connection within your home causing low voltage under load, but not always. I once made a service call to troubleshoot a home with dimming lights. In fact, the problem was so severe that their TV would switch off and the computer UPS alarm would sound any time they turned on their microwave! I quickly determined that more than one circuit was affected. This shifted my suspicion to the incoming service conductors. Sure enough, a visual inspection of the overhead service drop revealed a broken neutral at the utility pole! A call to our local power company got a crew out to make the emergency repair. The homeowner was happy to know it wasn't in their home, and wasn't their responsibility.
If this is your issue, look outside for broken wires leading into the home. Placing a call in to your power company might be your first course of action.