[DKAudio]

Like Tom said, when the motors start there is an incredible amount of in-rush current. It is so great for a small amount of time that your voltage in the house may actually decrease and available current for your other branch circuits will go down (hence the dimming lights) until everything reaches operating speed.

Many AC motors have start capacitors that give it a boost to start. This cap may be getting old and can't hold the charge it once used to. You can also verify that your conductors going out to the unit are of suffecient size.

Keep in mind though that motors are the heavest draw on an electrical system.