It was an inauspicious debut for CNN’s primetime news show, “Anderson Cooper 360″ in its new 8pmET timeslot. Cooper averaged 157K in the A25-54 demo and 502K in Total Viewers last week. By comparison the previous show, “In the Arena” hosted by Eliot Spitzer averaged 184K and 599K during Q2 in the time period, which, over the course of the quarter also included CNN’s New Hampshire GOP debate, two Presidential addresses and two special editions of Piers Morgan. It is the dead of summer, but considering the busy news week on Wall Street and in politics, it could be a rough climb for AC360 at 8pm — cable news’s most-watched hour.