In the thread "Weird Electric Hand Drill Problem", George Nuetzel suggested that the problem might be loose wiring in an outlet, especially if aluminum wiring had been used.
Unfortunately, this is an all too common occurrence, and continues to be a source of fires (heat generated by arcing between the loose wire and the screw contact.
If you have aluminum wiring, and you know how to do so safely, you need to ensure that the screws on all outlets, switches, and light fixtures are snug. As George said, tightening screws is just a temporary fix, so, while you are doing that, check to see if the device is approved for aluminum wiring. It will be clearly marked on the back. If it is not marked for aluminum conductors, it is not approved. Save yourself a fire: replace unapproved devices promptly. If you are not comfortable doing it yourself, find the money for an electrician. It could be your life or your child's if you try to ignore it.
Bruce