All of you experienced it, sooner or later. Even if you weren't graced with a good old front-loading NES yourself, you probably saw it at a friend's house while you were visiting to play Teenage Mutant Ninja Turtles II: the dreaded blinking power light. Sometimes the blinking light would even manifest itself as the entire game flashing on and off. "Stop!" you would scream in your annoyed, prepubescent voice. "I wanna play my game!" But it would just sit there, mocking you with the first half-second of the game's boot cycle, over and over.
The official cause of this problem, according to Nintendo, was dirty contacts on the game or in the machine. Temporary repair methods that various console owners came up with included the "isopropyl alcohol and Q-Tip" solution, the "temporarily mimicking a can of compressed air" solution, and the famous "hold reset while pressing the power button" solution (which actually didn't do anything to help this particular situation; it came about as the solution to a different problem, namely the battery backed-up RAM on cartridges being blown away on power down). But, try as one might to stave off eventual disaster, the problem would almost always get worse and worse until it would happen just about all the time, with any cartridge.
What Nintendo didn't tell people was that, although dirt and dust were certainly contributing factors, the blinking light problem was actually exacerbated by a design flaw. First off, the slot that the cartridge's edge connector inserted into was made out of cheap alloys, causing them to corrode over time. This, however, was only part of the issue; the other part dealt with the very nature of the NES' manufacture. Since the NES was released in the United States after the great videogame market crash of the early 1980's, toy stores were loathe to carry anything that called itself a videogame. Thus, Nintendo designed the NES to mimic audiovisual equipment of the era; the system was a "Control Deck", not a "console", and the games were "Game Paks", not "cartridges". The games loaded through the front of the unit, like a VCR.
And therein lay the crux of the problem. The user would load the game into the spring-loaded tray on the front panel, and push the game (and thus the tray) down, locking it into place. The force placed on the top of the game, however, caused the edge connector on the back to push up against the slot's fingers and bending them ever so slightly. This wasn't a problem for a few dozen, or even hundred, iterations of inserting and removing games; but after a few thousand times, those contacts would get bent just enough that they would no longer touch the cartridge as securely. (This also resulted in the strange behavior in some systems where a cartridge would work fine if you inserted it but did not press the tray down.)
Nintendo corrected this oversight in future machines by putting a good-old top-loading slot in the NES 2 and Super NES, at which point the original NES had rejuvenated the home videogame market and "consoles" were no longer taboo. But that doesn't help the huge installed base of old NES owners who are stuck with tons of old games and a defunct system.
The solution to this problem is to find a place that sells a replacement 72-pin connector for the NES -- not an easy task, although some specialty hobby and used game stores still carry them for a few bucks. Installing the new connector is relatively straightforward; there is no soldering required, as it simply plugs into its own slot on the NES motherboard. Once you do this, the system will be good as new, and it's absolutely astonishing how tight the NES will grip those games with a new connector installed! (Of course, the new connector will wear out slowly like the old one, but most new connectors are made from metals that won't corrode in the same fashion -- and, honestly, are you going to be shoving games into the system as much now as you did back then?)
So dig out the screwdriver, rummage through the couch for spare change, fix that system and return to those two-dimensional, square wave-bleeping star-crossed days of yore. You'll be glad you did when you pop in your favorite old game, turn it on, and say, "Wow, I remember this being a lot cooler as a kid."
But it'll be nostalgic just the same.
References: Beginner's FAQ at |tsr's NES archive (http://www.atarihq.com/tsr/index.html), troubleshooting guide at the official Nintendo webpage (http://www.nintendo.com/consumer/systems/nes/trouble_power.jsp), and plenty of infuriating personal experience!