Saving a corporation from an Arthur Andersenstyle meltdown aint enough to secure a CEOs job these days. So Michael Cherkasky is stepping down from troubled insurance conglomerate Marsh & McLennan Cos., which he helped avoid incurring criminal charges for alleged bid-rigging in its insurance brokering business. The former prosecutor struck an $850 million settlement in 2005 with thenNew York attorney general Eliot Spitzer. But Cherkasky couldnt overcome the weak performance of MMCs core risk and insurance business, which ran a 4.9 percent operating margin in the quarter ended September 30, versus rivals high-teens margins. Under Cherkaskys three-year reign, said nonexecutive chairman Stephen Hardis , MMCs financial performance has fallen far short of our expectations. Cherkasky was not available for comment.