Rising instability among some of the biggest US stocks is driving a measure of single-stock “fragility” to record levels, with the market increasingly vulnerable to whipsaw patterns among clusters of shares such as occurred in the dot-com bubble of the late 1990s.

Wall Street is taking a pause after running to the brink of a record following some mixed profit reports from big U.S. companies. The S&P 500 was flat in early trading Friday and hovering around the all-time high it set last month.