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 edged back from its all-time high on Friday, as U.S. stock indexes drifted following mixed profit reports from big companies. The S&P 500 barely budged and slipped by less than 0.1%, a day after rallying within 0.