Surprising analysts who had written off Blockbuster as a nearly extinct dinosaur, the video rental giant on Thursday reported a huge jump in profit during its fourth quarter. In an SEC filing, Blockbuster said that it earned $30 million against $8.3 million during the comparable quarter a year ago. If one-time costs including $13 million in severance payments, had been excluded, Blockbuster said, it would have reported $55 million in profits for the period. Shares in the video-rental company went on a roller-coaster ride Thursday, rising more than 9 percent following release of the quarterly report, then eventually plunging, closing down 10.2 percent as the overall market fell to a 52-week low. At midday trading today (Friday), Blockbuster shares were up 4 percent.