A glitch in a computer used in calculating the Dow Jones Industrial Average (DJIA) triggered a sudden drop in the blue chip index Tuesday afternoon and added to the concern caused by Tuesday's stock sell-off.

The problem began at 1:50 p.m Tuesday amid heavy selling and caused a 70-minute time lag in calculating the value of the DJIA, according to a statement from Dow Jones Indexes. A system that feeds market data to the computer calculating the DJIA suddenly began experiencing delays. "While the DJIA was still being calculated and disseminated, the calculator was not receiving the underlying component prices of the DJIA on a timely basis."

When the problem was identified, Dow Jones Indexes switched over to a back-up computer and the result was a sudden 200-point plunge in the DJIA as the system caught up with the latest market data. "This switch-over caused prices that were received during the latency period to be processed all at once, bringing the index immediately in line with its underlying component stocks.

"Dow Jones Indexes is continuing to investigate the latency issue to correct the root cause of the problem."

The company said it expected normal operation of the DJIA Wednesday using its redundant market data system, and as of 3 p.m. (EST), there had been no reports of further problems.

Tuesday witnessed the biggest one day slide of the New York Stock Exchange and S&P 500 since Sept. 11 2001. The Dow Jones Industrial Average lost more than 416 points in the selloff, which also hit some of the nation's online brokerages. Keynote Systems Inc., a company that monitors the performance of Web sites worldwide, saw a 25 percent decrease in the number of effective transactions it could execute on various online brokerages from 1:30 p.m. EST until the market closed at 4 p.m., said Abelardo Gonzalez, Keynote's Web performance manager.

The Gomez Brokerage Benchmark, another Web performance monitoring firm, noted similar "catastrophic slowdowns" at some of the largest brokerage sites around 3 p.m Tuesday.