Extending their up and down trend, U.S. shares reversed sharply late Thursday ending lower on bad news from the networking sector. The S&P 500 slipped 17.02, or 1.5% to 1080.96, while the Nasdaq Composite lost 59.23, or 3.3% to 1716.34. The Dow industrials shed 106.49, or 1% to 9834.68. Dealers said volume was solid for the second straight session. Ciena Corp fell after lowering second quarter estimates due to weak customer spending. Dealers said the news shook assumptions for a solid mid-year recovery in the technology sector. Networking companies sank sharply.