NEW YORK (TheStreet) -- For the past two years, Pioneer Cullen Value (CVFCX) - Get Report has been disappointing. The fund lagged 73% of competitors in 2009 and 92% in 2010, according to Morningstar.

Should shareholders sell? Probably not. Veteran portfolio manager Jim Cullen is sticking to his investing discipline, and the long-term track record remains superb. During the past 10 years, the fund has outdone 98% of its large value peers.

Pioneer Cullen's recent slump is hardly unusual, according to a recent study by Baird, a money manager and mutual fund company. Nearly all managers suffer periods of underperformance, the researchers found. Baird looked at top-performing funds, a group that had outdone the benchmarks during the past 10 years while recording less volatility.

Of the top funds, 85% had at least one three-year period when they lagged the benchmark by 1 percentage point or more. All the funds in the study dropped below their peer groups for at least one year.

Funds that slumped often rebounded quickly. In the study, Baird looked at top-performing funds that had suffered a slump and received a rating downgrade from Morningstar. During the next three years, the funds on average rebounded and outperformed their peers.

Baird concluded that to succeed over the long term, investors must patiently hold funds through good times and bad. But all too often shareholders follow a self-defeating pattern. The investors buy after a fund has recorded a hot streak. When the inevitable slump occurs, the shareholders lose patience and sell at the wrong time. Consider

Yacktman

(YACKX) - Get Report

, which returned 12.5% annually during the past 10 years, outdoing 99% of large value peers.

Portfolio manager Donald Yacktman buys undervalued stocks with strong balance sheets and rich cash flows. During the market downturn that began in 2000, Yacktman excelled. But with stocks rallying in 2004, he slipped into the bottom quarter of the standings and stayed there for three consecutive years. Figuring that the manager had lost his touch, angry shareholders dumped the fund. Assets in the portfolio dropped from $441 million in 2005 to $293 million in 2007.

Never wavering from his style, Yacktman roared back. The fund outdid 98% of competitors in 2008 and 99% in 2009. Since then, investors have been pouring into the fund, which now has $4.2 billion in assets. Many of the new investors have been attracted by the recent performance. But soon enough the patience of the shareholders will be tested as Yacktman goes into one of its periodic slumps.

Make no mistake, there are cases when you should sell. If the manager changes or the fund switches strategies, then there may be little reason to stick around. You should also lose patience when a slump lasts longer than three years.

Before making any decision about selling, start by trying to determine why the fund may be slumping. In many cases, funds turn cold because their styles are out of favor. Consider Pioneer Cullen Value. Portfolio manager Jim Cullen favors stocks with low price-to-earnings ratios and solid growth prospects. The strategy excelled during the downturn of 2008, but the approach has lagged since the market began rebounding in March 2009.

The poor performance is not surprising because the portfolio holds high-quality blue-chips, including

Microsoft

(MSFT) - Get Report

and

Kraft Foods

(KFT)

. During the rally of the past two years, the market has favored shaky small value stocks, while Cullen's blue-chips have lagged. Instead of dumping the fund, shareholders should wait. Eventually blue-chips will come back in favor.

Another fund worth holding is

Davis NY Venture

(NYVTX) - Get Report

. The fund was clobbered in 2008, and it has underperformed peers in three of the past four years. But the long-term record remains compelling. During the past 10 years, the fund returned 4.2% annually, outdoing 75% of large blend peers.

Portfolio manager Chris Davis looks for dominant companies with solid returns on equity. He aims to buy when the stocks have slipped out of favor. The fund has long held a big stake in financials. That proved to be a disaster in 2008. The fund still has big holdings in dominant financials, including

American Express

(AXP) - Get Report

and

Berkshire Hathaway

(BRK.A) - Get Report

. Those should continue increasing earnings as the economy recovers.

Recognizing that actively managed funds can suffer periodic slumps, many investors have concluded that the solution is to buy index funds. But index funds suffer cold streaks. In two of the past four years,

Vanguard 500 Index

(VFINX) - Get Report

, which tracks the

S&P 500

, has lagged most members of the large blend category. As that record shows, stocks are unpredictable, and there is no easy way for investors to avoid suffering periods of disappointing results.

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Stan Luxenberg is a freelance writer specializing in mutual funds and investing. He was executive editor of Individual Investor magazine.