PC keeps power cycling every 5 seconds

So, last night my PC decided to throw a complete tantrum. It froze up on me first, forcing a manual restart, only to freeze again during the boot sequence. After another reboot, it entered this endless loop where it just cycles power every four or five seconds—the fan spins up, it dies, waits a few seconds, and then does the whole thing all over again. I actually left it sitting there in that loop overnight, and after about an hour of this rhythmic death throes, it finally decided to behave itself. It worked fine this morning, but now that I’m back home, we are right back where we started with the constant rebooting. The rig is two years old; I had it custom-built at Micro Center back in September 2012. I followed the hardware recommendations provided by the folks here on the forums when I was putting it together. Currently, I'm running Ubuntu 14.04 LTS on it. To give you the full rundown: I've got an Intel Core i3 processor paired with an MSI R7750 1GB graphics card, an AsRock B75 Motherboard, a Corsair 6 GB SSD Force3 for the OS, and an EVGA 450 W1 power supply. Does anyone have any inkling as to what might be causing this madness? I am desperately hoping it isn't the GPU giving up the ghost; that thing cost me a small fortune...

Start with the power supply. Maybe borrow one from a friend or a neighbor and see if that does the trick.

Just like Carpet mentioned, I’d start by looking at the power supply—try to run some tests on it if you can. If the PSU isn't the culprit, you might be looking at a dead motherboard (that’s exactly what happened to me when I had these same symptoms), or potentially a fried graphics card (let's pray it's not that). Honestly, though, it's pretty weird that the PC actually booted up just to freak out again. To me, that screams power supply issues. Maybe double-check the connection between the motherboard and the PSU? That's a common spot where things go sideways.

It occurred to me that this might actually be a power supply issue, considering the whole system crashes right when the fan starts spinning up. That’s usually that peak moment where the power draw spikes, isn't it? My first instinct was to check if a cable had shaken loose from its socket, but everything seemed perfectly seated. I just finished cracking the case open and gave the entire interior—including the CPU and GPU fans—a thorough pass with a vacuum to clear out the debris. Surprisingly, it seems to be running fine for the moment. Now, we wait. If it decides to start acting up again, I'll probably just borrow a PSU from a buddy to see if that fixes the problem.

Well, good luck then. I really hope it’s nothing too serious 😉