New computer, power supply, and GTX 550 TI: computer shuts itself off

I know there are many threads with this similar problem; however, all of the ones I have read involve people putting a new card in an old system. This is a brand new PC which was working fine. I upgraded the power supply to a Corsair CX500, which according to specs and popular opinion, should be more than enough to power the card. So, if I install the card and use it as a video out, the computer will stay on for a couple of minutes until it shuts off the first time. Then it will only power on for a few seconds. It seems like an overheating issue to me. My question: Do you agree that its an overheating issue? If so, WHY? There is no dust, no past shoddy heatsink work haunting me, and I've tried it leaving the case open for maximum airflow and it still does it. Do you think the card is defective?

Typical troubleshooting questions answered:

Is it powering off under normal conditions or under load? Unless you consider displaying 1024 x 768 resolution and the Windows Desktop a load   , then no, it is just powering off under normal conditions.
All fans working? Yes.
All cables/cards/chips seated correctly? Yes (unless it's the chip/heatsink/fan on the card itself... I'm not messing with those b/c it's new)
Does computer go back to working normal with graphics card removed? Yes.
Have you run a diagnostics utility to check the fans/temp/etc of the GPU and system in general? No, because it won't stay on long enough.

I sincerely appreciate any help you all can give. I am just trying to do a favor for a family member and help them out, so thanks in advance for helping them out as well.