Momentum investors are stock players who get behind companies that have the wind in their sails. Contrarian investors typically pick up the cigar butts the market has tossed aside. So what do you call investors who turn against winners? Sourpusses? Shorts?

Over on Motley Fool CAPS, we sometimes call them the savviest investors around. Not only does the 75,000-strong investor-intelligence community rate thousands of stocks every day, but the players themselves get rated, too. The best of the lot -- what CAPS calls All-Stars -- consistently outperform their peers over time and are assigned ratings of 80 or greater.

When an All-Star player sours on a top-rated stock, perhaps we should take notice. Perhaps the player's found a chink in that highflier's armor, or a question mark in its financial footnotes. Or maybe it's just a hunch. That's why these tables aren't lists of stocks to buy or sell -- just starting points for further research.

Here's a list of stocks that some All-Stars have recently given the thumbs-down to:

Company

CAPS Rating (out of 5)

1-Year Return

CAPS All-Star

Player Rating

Companhia Vale do Rio Doce (NYSE:RIO)

*****

131.5%

dwot

99.97

Southern Copper (NYSE:PCU)

*****

104.9%

Bridgeboy0

98.69

GigaMedia (NASDAQ:GIGM)

*****

77.8%

mavericktrading

99.65

Nvidia (NASDAQ:NVDA)

*****

30.7%

humvee5000

99.34

NYSE Euronext (NYSE:NYX)

*****

(20.0%)

mitch48183

87.72

More than 8,450 investors have rated these stocks, and an average of 99% of them are bullish on their prospects -- and a like number of All-Stars also think they'll outperform the market. What might have turned some of CAPS' top players against these otherwise widely admired companies?

Envisioning Nvidia
High-end computer users and gamers have long held Nvidia's graphics chips in high regard. The company's products have improved the quality and performance of games and of desktop and laptop computers, as well as handheld devices and mobile phones. Nvidia sits atop the mountain, but Intel (NASDAQ:INTC) has decided to enter the fray; that company will be competing head-on with both Nvidia and Advanced Micro Devices (NYSE:AMD) through its ATI acquisition.

In marking down Nvidia's expectations, humvee5000 is expressing a belief that ATI can't possibly do any worse than it has (so it can only move up), and that going head-to-head with Intel is a loser's game:

Hit its peak -- ATI can not possibly do as badly as it has done even though Hector is the boss at AMD. Since NVDA has already taken about half the share that ATI had and much of the high end stuff -- there is diminishing ability to take more and the multiples already price in great growth. Also in time Intel is going to be going toe to toe with NVDA and whoever does that loses!

However, other All-Stars, such as icon149 -- who has a 99.57 player rating -- think the next round of industry growth will push Nvidia still further ahead:

Tech is about to take off again, Nvidia is best in [breed], Vista is coming, PS3 will add an ever increasing and stable revenue stream as Sony ramps up production.

There's something to be said for the push the graphics chip maker will feel from game consoles, and the Wii certainly seems to be the must-have one this season.

Make lemonade from lemons
We know both sides here, but Motley Fool CAPS is more than what some pros think, even if they're All-Stars. It's where we invite you to share your thoughts and insights and add your voice to the debate. Go ahead, have your say. We're eagerly waiting!

Nvidia is a recommendation of Motley Fool Stock Advisor. GigaMedia is a Global Gains selection, while NYSE Euronext is a Rule Breakers pick. Intel is an Inside Value recommendation. You can try any of our investment services free for 30 days by clicking here.

Fool contributor Rich Duprey owns shares of Intel, but does not have a financial position in any of the other stocks mentioned in this article. You can see his holdings here. The Motley Fool has a disclosure policy.