While major stock-market indices continue to hover near record highs — the S&P 500 stands 3 percent below its peak for example — fewer stocks are keeping the market aloft.

"Just a few companies are driving the gains in major U.S. stock indexes this year, raising fresh concerns about the health of the market’s advance," writes Wall Street Journal reporter Dan Strumpf.

More than half of the Nasdaq Composite's 6.3 percent gain in 2015 comes from just six stocks — Amazon.com, Google, Apple, Facebook, Netflix and Gilead Sciences, according to brokerage firm JonesTrading.

And for the S&P 500, Amazon, Google, Apple, Facebook, Gilead and Walt Disney have provided more than all of its 0.3 percent climb.

This could presage a sharp drop for the market as occurred after the peaks of 2000 and 2007, periods that also saw the late gains sparked by only a few stocks.

"It can go one of two ways: either the whole market tanks, or the market broadens out," Scott Migliori, manager of AllianzGI Focused Growth fund, told The Journal. "The rubber band has been stretched too far."

The S&P 500 hasn't suffered a 10 percent correction since October 2011. Since 1946, the average period between corrections has been about 18 months.

Meanwhile, Mohamed El-Erian, chief economic adviser for Allianz, says global central bank easing is no longer enough to keep pushing financial markets higher.

"What the market is telling you today is, 'You know what? That is no longer sufficient to maintain prices at a high level.' You need something more, and that something more is fundamentals," he told CNBC.

So what should investors do? Take out money from public markets, and put some of it in cash to provide dry powder that can be deployed when markets drop, El-Erian says.

As for the rest, "don't give up on some really exciting opportunities that are happening in the start-up world, in the private equity world," he said. "There's a lot going on, especially in technology."

Of course many of us aren't wealthy enough to invest in private equity funds or in venture capital funds that back start-up tech companies. You can invest in the stock of some private equity managers like Blackstone, the world's largest. In addition, there are mutual funds that try to replicate private equity investments and ones that invest in venture-backed tech start-ups.

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