"The bigger they are, the harder they fall." It's the worst nightmare of every investor in today's market -- buying a rocket stock just before it takes a nosedive.

Now I readily admit that sometimes, stocks rise for a reason. But sometimes, the rise becomes the reason. No matter how often we caution them not to, investors do have a habit of buying "hot" stocks, and trusting momentum to keep 'em moving upwards.

Problem is, if the price goes up too much, even a great company can turn into a lousy investment. Below I list a few stocks that may have done just that. Stocks that, according to the smart folks at finviz.com, have more than doubled in the last half of '09, and just might be ripe to fall back to earth.

Stock

Recent Price

CAPS Rating
(out of 5)

Finisar (NASDAQ:FNSR)

$10.30

****

Taseko Mines (NYSE:TGB)

$4.87

****

Vonage Holdings  (NYSE:VG)

$1.61

*

US Airways  (NYSE:LCC)

$5.24

*

UAL (NASDAQ:UAUA)

$13.21

*

Companies are selected by screening for 100% and higher price appreciation over the last six months on finviz.com. Five stars = highest possible CAPS rating; one star = lowest. Current pricing provided by Yahoo! Finance. CAPS ratings from Motley Fool CAPS.

Last year's last-half market rebound was kind to these stocks, to be sure. But now that Mr. Market has reached out, tapped us on our collective investing shoulder, and reminded us that stocks can go down as well as up, the question becomes more urgent: How many of these stocks can keep on winning?

Our 145,000 CAPS members don't seem too optimistic about most of these companies continuing last year's winning streak much farther. There is one stock, however, that we believe can keep on powering higher:

The bull case for Finisar
Finisar! Now that's a name that's a blast from the past. Over on CAPS, most Fools had durn near forgotten the stock existed until just recently. In fact, prior to this year starting, the last we heard anyone talking about the stock was when CAPS All-Star pennysplants pointed out (back in the summer of 2008) that the market for high-speed data transmission was likely to "grow exponentially (although that may still be sometime down the road!)"

"Down the road" indeed. In fact, it would be another 18 months before the sector attracted enough interest for Fools to begin talking about Finisar again. And as you might expect, it was one of our very best investors who first noticed -- fellow All-Star investor ElginBaylorkt -- who pegged the stock as: "High growth. Moderate risk. Cheap, cheaper than it will stay even if growth is slower than projected" earlier this month.

Growth: Past and future
Once upon a time, Finisar was a big name in the network optics industry, a rival to companies such as JDS Uniphase (NASDAQ:JDSU) and Cisco Systems (NASDAQ:CSCO). And having avoided a near-death experience, the stock's gone on to post amazing gains -- 163% -- over the past 52 weeks. That's enough to wake anybody up, and as ElginBaylorkt's note shows, it is already attracting attention. But now that we're awake to the fact of Finisar's existence, the question arises: Should we care?

Not meaning to burst any bubbles here, but I'd argue that no, we should not care. Sure, I'm pleased as punch that some investors out there have enjoyed gains from the stock's revival. But from where I sit, there's little chance that the rest of us will see any more. For even as the stock has gotten stronger, Finisar's business has gotten weaker.

It's been more than a decade since Finisar last posted an annual profit, you see. And as far as cash flow goes, well, the company seemed to be improving for a while, but cash flow fell off a cliff last year, and is actually running negative this year.

Now, maybe Finisar can grow its way back to health -- analysts, at least, expect to see it post nearly 15% annual profits growth over the next five years. But if history is any indication, this company is just as likely to fall on its face again, as to suddenly reverse years of subpar performance and emerge as a credible competitor to the many, many better-run companies in this industry.

Foolish takeaway
Long story short, I'm not optimistic about the company's chances (or had you noticed?) But that doesn't mean you must agree with me. To the contrary, if I'm reading this story all wrong, I'd appreciate it if you could point out the errors in my thinking.

Here: I'll even give you a soapbox to stand on. Shout away.

Fool contributor Rich Smith does not own shares of, nor is he short, any company named above. You can find him on CAPS, publicly pontificating under the handle TMFDitty, where he's currently ranked No. 715 out of more than 145,000 members. The Fool has a disclosure policy.