NEW YORK (CNNMoney.com) -- Was that it? The stock market just closed out the best third quarter in 9 years, defying expectations for equities to stall or more likely, slide.

But the question for investors going into the fourth quarter - typically the best of the year - is whether the advance can continue.

After a summer rally, the S&P 500 index stands just below a 5-1/2 year high and the Dow is just below its all-time high, not seen since Jan. 2000. The Nasdaq composite is up modestly year-to-date, but remains well below its record high.

October tends to be a volatile month historically and with the major stock gauges hitting up against multi-year highs, next week could be particularly testy.

"The economic data and earnings will ultimately determine whether we see our traditional fourth quarter rally or whether we've in fact moved it up into the third quarter," said Paul Mendelsohn, chief investment strategist at Windham Financial Services.

The S&P 500 gained 5 percent in the third quarter, which ended Friday. That was the best third quarter performance for the broad index since 1997.

What fueled it? Oil prices tumbled more than 20 percent after peaking in July above $78 a barrel. Geopolitical dangers seemed to cool. Bonds rallied, lowering yields and helping stocks. And the Federal Reserve paused - and possibly ended - its more than two-year old interest rate-hiking campaign.

On one hand, Mendelsohn said, these factors and the momentum of the market could fuel further gains. But on the other hand, the market looks vulnerable to a selloff, after such a stellar quarter.

On top of that, it's unclear just how much the economy is slowing, and "a slowing economy is equated to rising risks for stocks," said Alan Gayle, senior investment strategist at Trusco Capital Management.

Last week, a report showed second-quarter gross domestic product growth slowed to a 2.6 percent annualized rate, missing economists' forecasts. Next year, growth is expected to sink to around a 2 percent rate from a projected 3.5 percent rate in 2006.