MasterCard ( MA) shares dropped 12% after it warned that last year's increase in operating margin of nearly 3% might be hard to repeat next year. Shares were tanking $13.76 to $100.98. New Century Financial ( NEW) fell again after an analyst with Standard & Poor's cut the subprime mortgage lender's credit rating to BB- from BB, citing rising bad-debt trends. Shares were sinking $2.23, or 11.6%, to $17.01. Kingsway Financial ( KFS) slumped after the Canada-based insurer's fourth-quarter profits fell by more than half to $16.8 million, or 30 cents a share. Analysts with Ferris Baker Watts and CIBC World Markets downgraded the stock. Shares were falling $1.70, or 8%, to $19.64. New York Stock Exchange ( NYX) tumbled after a Goldman Sachs analyst cut the exchange's rating to neutral from buy, citing slower-than-expected volume growth on its new hybrid trading system. Shares were down $4.92, or 5.1%, to $92.32. Aon ( AOC) rose after the health insurer said it earned $189 million, or 67 cents a share, from continuing operations in the latest quarter. The profits exclude items. Analysts were expecting 63 cents a share. The Chicago-based company was gaining $1.58, or 4.3%, to $38.01.