Is there a cordless phone base station in that room ? If so try moving it into another room and see if that room becomes the dead zone or it stops the dead zone in that room . I've got a Uniden that wreaks havoc on the wifi all over my house and if wifi is being used causes static in the phone when in use. Many routers and cordless phones run 2.4 ghz and can interfere with each other. Sometimes the transformer on a halogen or mercury bulb lamp can cause it, though they usually cause monitors to act up if too close.
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