I built this computer earlier this year (early spring), and it has been running fine since (i had absolutely no problems with the initial build). Now, however, it has started having trouble staying on. It will randomly turn off, often quickly after i turn it on (before boot or even POST, or during one of these processes). I am suspecting the power supply, but there are a few odd pieces to the situation.

I know it isn't the CPU or RAM (if it were either of those, it woouldn't boot, right?), and i doubt it is a motherboard failure (same). I have been able to boot successfully into windows, and mostly everythign seems fine. The only exception is that i have no network connection (which could easily be unrelated), and attempting to run 3D games (i have tested HL2 and Crysis) will cause it to shut down. When it turns off it issues a single beep, no matter where the computer was in the boot process or what i was doing.

So I am strongly suspecting the power supply, but the fact that specific (graphics intensive) applications cause shutdown also points to the graphics card. The beeping bothers me, because it seems to say that the computer has some idea what is wrong, but can't tell me. Any help with this is much appreciated, it was an expensive machine that i was hoping not to have to mess with or replace for a very long time. I can get detailed system specifications if they're needed.

Loading up graphics intensive games will cause the gpu to need more power. When it tries to draw this power from the PSU it might overwhelm it and cause it to power off.

If you have another powersupply handy to test this theory I would recommend giving it a go (maybe take a known good psu out of another computer and try it out real quick)

Very true the The PC Requires a combination of things to run well under load. CPU Graphics RAM all need a high wattage PSU to run well. Best bet is to look into the PSU first

Its overheating

Things have taken a turn for the worst. I had sent out the power supply (and case, because they came together), and have now gotten a brand new one back. The same problem persists.

Is there any way to diagnose this? I'm thinking it could be the RAM, CPU, or Motherboard...

if your mobo has onboard graphics then remove the vga card and try booting it up, or download nTune and run the gpu stress test while monitoring the temps. if it jumps too high too fast, or is simply idling too hot you may have found the problem.

and you're going to need to give us your system specs (psu wattage, cpu, gpu, mobo...)