LANCASTER - The Texaco gas station on Challenger Way and Avenue K is back open and customers are slowly returning to the pumps, but no one has figured out just what contaminated the regular unleaded four weeks ago. More than 100 people have reported problems arising from bad gas, according to the station's StarMart manager, Bob O'Kelley. The station closed on Sept. 27 and reopened on Oct. 10. "It's picking up slowly," O'Kelley said. "We're getting our regulars back. We have the same customers every day because we're in the neighborhood. It's not like we're on the freeway, a car...