Erratic moving light beneath a Delta Flight out of Dallas.I was on a Delta AL flight from Dallas to New Orleans, climbing out of DFX over the eastern suburbs of Dallas. I was seated well aft of the wing on the starboard side of the plane with a clear view of the ground. I noted a light shining on the ground at about our 4 oclock position. I at first thought it was a police helicopter shining it's searchlight on the ground. It was traveling in essentially the same direction and speed as our plane (B737-800). I suspected it may be a light from our aircraft, but eliminated that possibility when the light went under some scud...and the scud was back-lit. It would reappear on the next opening in the scud. That happened several times. We were in clear areas for long periods of time when it would look like it would do zig zags at right angles to our path and then return to the same direction as our path. None of these manuevers were in tune with aircraft attitude changes, except for it's general direction. I called it to the attention of a man seated next to me and he too followed it's progress and witnessed it going under a thunderstorm cell and reappearing on the other side of the cell as we passed by. He too had no possible explanations for the light. Once in a while it would flare out to a wider area like a flashlight being raised slightly, but most of the time it gave the appearance of a cars headlight shining down a wooded road..with the flashing as trees obstructed it. The squared nature of its movements would also give the impression of a car going on checkered board roads. But, we were now moving at a ground speed of 525+mph and at 30,000+ft. The light would not be as bright going through towns, but one could follow it's progress through them in the same general direction and occasional zig zag. It would appear more brightly out in the open country. We came upon a real strong band of thunder storms and the light went under them and except for one break was not sighted again on the flight. Clouds obscured the ground on the descent into MSY, ! but when the ground did appear, the light was not there again. All in all, probably the most interesting flight of a flight filled life. The thunderstorm band was spectacular, it was the night of the devastating tornados just NE of our flight path.((NUFORC Note: We have no idea what the passengers witnessed. If the report is accurate that more than one passenger witnessed the alleged light, and that it did, in fact, disappear behind clouds, then it could not have been a reflection from the inside of the aircraft. Had it been a daylight flight, a possible explanation of the sighting might have been the phenomenon known as "Broca's Spot," in which a spot is seen on the ground where the plane's shadow is cast, but this phenomenon does not occur at night, obviously. We are confused by this one. PD))