Planet with a sun that is subjected to a coronal mass emission. The world is peppered with tornado-like anomalies and geomagnetic storms. These storms are what power an Ancient time-loop device on the world.
Originally the Ancients had settled this planet as one of their bases. When a great plague, perhaps the great plague arrived they built a time machine to travel back and fix the problem. Unfortunately the device never worked to its full potential. They shut it off and the planet was engulfed in the plague, killing any un-ascended Ancients that remained.
HOME TO - Uninhabited
FIRST APPEARED - Window of Opportunity
Window of Opportunity - SG-1 and the alien Malakai meet on P4X-639 to study the geomagnetic storms and a mysterious device.