As the economy stumbled in the second quarter, magazine ad sales fell more than 8 percent, with the steepest drops in ads for vehicles and for computers and related equipment, according to a report released Thursday. The report, compiled by the Publishers Information Bureau, shows the industry’s troubles growing worse. Ad pages sold in magazines in the United States declined slightly last year, 0.8 percent, then fell 6.4 percent in the first quarter of this year compared with the period a year earlier, and 8.2 percent in the second quarter. For the first six months of the year, automotive ad...