Even on the market's worst days, buyout news and other short-term forces can send individual stocks up by 10%, 25%, even 50%.

For example, shares in solar stocks bobbed up and down wildly as we approached and rang in the New Year, including ReneSola, which inexplicably shot up more than 33% in a day.

But beyond less-predictable events like that one are stocks with fundamentally compelling reasons for recent momentum. The trick is to find those stocks. That's where Motley Fool CAPS comes in.

The story behind the story
CAPS is no crowd of lemmings. Its best-performing members' opinions do more to shape each company's rating than the picks of their poorer-performing peers. Let's use the collective wisdom of more than 125,000 CAPS members to filter out the noise and find companies offering strong momentum.

We'll use CAPS' handy stock screening tool to quickly zero in on companies with a stock price increase of at least 35% in the past four weeks, a market cap of greater than $100 million, and a beta of less than 3.

Below is a sample of stocks our screen returned. If you like, run this screen yourself; just keep in mind that results will be updated as the market changes:

Company

CAPS Rating
(Out of 5)

4-Week
Price Change

GigaMedia (NASDAQ:GIGM)

*****

37.8%

Alcoa (NYSE:AA)

****

48.6%

PotashCorp (NYSE:POT)

****

44.9%

Source: Motley Fool CAPS. Price return from Dec. 5 through Jan. 2.

Betting on a recovery
While it's hard to find shares of any Chinese company that have escaped punishment recently, online gaming firm GigaMedia has been hit much harder in the past year than rivals like NetEase.com (NASDAQ:NTES) and Shanda Interactive, sending its share price down to the single digits late in the year. But the company remains profitable, and it posted double-digit earnings gains in the third quarter. With no debt and more than $100 million in cash, many investors feel the company is virtually being given away at these prices.

GigaMedia isn't holding back in developing new business, either. The company recently announced its plans to expand its online gambling empire into the sports betting world, partnering with Victor Chandler International Group, one of the world's oldest and best-known online gambling companies, to launch Everest Bets. GigaMedia's Everest Poker is one of the world's most popular poker sites, and Victor's sports betting business already accepts more than 2 million sports betting calls per year in more than 160 countries, providing both companies significant cross-marketing benefits.

Given its low valuation in the face of plentiful future growth potential, 98% of the 2,025 CAPS members rating GigaMedia expect it to outperform the market.

Feed the need
Investors have had quite a rollercoaster ride with fertilizer stocks like PotashCorp and Mosaic (NYSE:MOS) in 2008. The demand cycle that had been pushing commodity prices through the roof started to slip late in the year, sending shares plummeting. The global economic crisis cut demand for crop nutrients such as potash, nitrogen, and phosphate, and PotashCorp reported weaker sales in all three during the fourth quarter. It also lowered its full-year earnings outlook, but CEO Bill Doyle said the company expects demand to pick up as 2009 progresses, since food production and the necessary fertilizer applications can't be delayed indefinitely.

While potash prices stayed stronger over the past months thanks to decreased production, supply has been high for nitrogen and phosphate, leading companies like Terra Industries (NYSE:TRA) and Mosaic to now cut back on production. While nitrogen and phosphate have had constrained margins, PotashCorp expects gross margins for potash to triple compared to 2007. Despite the current volatility, many investors look for a better 2009 for fertilizer stocks like PotashCorp and highly rated Agrium (NYSE:AGU). In PotashCorp's case, nearly 96% of the 4,274 CAPS members rating the firm remain bullish on its prospects to beat the market.

And you?
What's your story? Whether you buy the tale of a stock that's soaring or souring, your own research is more important than collective opinions. But these collective opinions can make your due diligence a whole lot easier.

Add your take on these or any of the nearly 5,400 stocks that our 125,000-plus members have covered in Motley Fool CAPS. It's totally free to be a part of the community, and the payback is more than worth it.

On Jan. 12, 2009, Fool co-founder David Gardner, Jeff Fischer, and their Motley Fool Pro team will accept new subscribers to their real-money portfolio service. Motley Fool Pro is investing $1 million of the Fool's own money in long and short positions in a range of securities, including common stocks, put and call options, and exchange-traded funds (ETFs). They also incorporate proprietary CAPS "community intelligence" data into their research. To learn more about Motley Fool Pro, and to receive a private invitation to join, simply enter your email address in the box below.

Fool contributor Dave Mock has his own story, but there's no "happily ever after" at the end of it. He owns no shares of companies mentioned here. GigaMedia is a Global Gains selection. NetEase.com, Shanda Interactive, and GigaMedia are Rule Breakers recommendations The Fool's disclosure policy has the momentum of a freight train, but it can stop on a dime.