It may be silly to suggest stocks are running out of steam as they strike new highs, but that may be exactly what is happening, according to Art Cashin, director of UBS floor operations at the New York Stock Exchange.
The hit a new intraday all-time high of 2,192 on Monday, extending gains from the previous week, when the Dow, S&P 500 and Nasdaq all closed at record highs on Thursday for the first time since December 1999.
"There is some risk that we're approaching stall speed here, that they did it in such an incremental fashion that it really hasn't swept up everybody's attention," Cashin told CNBC's "Squawk on the Street" on Monday.
To be sure, Cashin sees the S&P likely rising to 2,200 if it breaks through resistance levels between 2,191 and 2,194. To do that, stocks need to get a bounce on resurgent crude futures, which were trading at a nearly one-month high on Monday on signs that producers may take action to put a floor under oil prices.
However, 2,200 could possibly define the upper end of the S&P's range for some time, much as 2,100 did for more than a year, he said.
The market will be awaiting data due from the U.K. in the coming weeks that will give investors a read on the Brexit vote's impact on the economy, Cashin added. The story of the day, he said, is that the British 30-year bond is now yielding less than the U.S. 10-year Treasury.
"If you had told people that months ago, no one would have believed it. Of all the central banks with a bazooka, the Bank of England certainly had one," he said, referring to the BOE's interest rate cut earlier this month.
More immediately, Cashin said he'll be pouring over minutes from the Federal Open Market Committee's July meeting when they are released on Wednesday. He said he wants to see how much time Federal Reserve policymakers spent discussing the lack of pressure on inflation, a key consideration for a second interest rate hike following an initial quarter-point increase in December.
Beware the 'post-Brexit halo' and 'fear the talking Fed'
Earlier Monday, Allianz Global Investors' Kristina Hooper also said the rally looks like it is starting to peter out, and investors should brace for bouts of volatility that could upend a vulnerable rally.
"There are certainly a lot of question marks around it right now. Certainly stocks are the most attractive asset class among a sea of relatively unattractive asset classes," she told CNBC's "Worldwide Exchange."
"It certainly is vulnerable just because it's not being driven by fundamentals."
For that reason, Hooper sees more downside risk than upside potential for equities in the event of macroeconomic shocks. Allianz is forecasting greater volatility through year-end, with the frequency of shocks, including those related to the U.S. presidential election, likely to increase, she said.
Part of what's driving the rally is a "post-Brexit halo," the strategist said. Following Britain's vote to leave the European Union, investors believe the Federal Reserve and central banks around the world are likely to keep interest rates low or negative, making stocks look attractive compared with bonds, she said.
"That can only take you so far, and while it seems unlikely that the Fed raises rates in September, it's certainly on the table," she said.
Like Cashin, Hooper said she is also anticipating the FOMC's July meeting minutes for a more holistic view of the Fed's thinking. Presently, the Fed is sowing confusion because regional Fed presidents and members of the FOMC board of governors are sending mixed signals in public comments.
"There's interpretation, misinterpretation, differences of opinions, and that's all aired out in the public. I like to say fear the talking Fed," she said.
Bank of America Merrill Lynch is also expecting greater "episodic volatility," or brief bouts of extreme volatility, said Mary Ann Bartels, head of portfolio strategy at Merrill Lynch Wealth Management.
While stocks remain on an upward trend, the CBOE Volatility Index, or VIX, has recently flashed a warning signal by creeping above 10, according to Bartels.
Despite low U.S. economic growth, U.S. stocks still look like the best option, especially considering the relative strength of the country's banking system, she said.
"Trends are higher. It doesn't mean that we'll be at new highs by the end of the year, but there's no other place," she told "Squawk Box." "There is no other alternative."