(redirected) Gaming computer is overheating to 90 degrees celsius

Hello everyone.  My gaming computer, purchased in April 2013, overheats and reboots when the graphics come under heavy load.  Here's the overview:

Computer

Alienware X51 PC running 64-bit Windows 8.1 (although the same problem happened with Windows 8). I'm running an Intel Core i7-3770 CPU @ 3.40GHz, 16 GB of RAM, DirectX 11. My graphics card is an NVIDIA GeForce GTX 660 with Integrated RAMDAC. No problems have been found on this card with dxdiag. The PC is about a year and a half old.

Problem

For the past 4 months, when I'm playing World of Warcraft, the screen goes black, the computer sounds like the fan is on turbo-mode, and I get a "Monitor going to sleep" message. For a while the computer would just shut down. I've updated my NVIDIA drivers and now its about 50/50 whether the PC itself just shuts down or it occasionally will reboot with an NVIDIA error, only to have the same error about 30 seconds later. My computer is fully updated, my virus software is updated and scans show I'm virus free. I don't really use the internet or download things on the computer, and I don't use email on the computer, so it's probably not a virus or malware.

This only occurs when I'm playing WoW, and happens when there's a noticable load on the graphics (raid environments, etc).  I am able to play several other graphics-intensive games like Rust, Rift, League of Legends, etc, without issue.

What I did

I posted to another tech forum and was told it was likely my GPU that was overheating.  I was told it could be the heatsink or the drivers, perhaps even the thermal paste.  I opened the computer and looked inside, and it looked like the heatsink for the graphics card had a ventilation fan that was not working when powered up. I brought the computer to Best Buy's Geek Squad about a week ago (no idea where else to bring it - the computer is no longer under warranty with Dell) and they weren't really sure what was going on, but they said the fan attached to the heat sink should always be on while powered. So I figured that the graphics card was just burned out.

I ended up buying a brand new NVIDIA GeForce GTX 660 (the same one) and paying them to install it for me.

Well, the same exact problem is still happening. When I run a GPU-Z sensor software, the log reflects that my GPU temp is steady in the high 80's and topping out at 90 degrees celsius before it shuts down (while gaming).  The GPU sits at around 30-60 celsius during other games. All the drivers on the new card are up to date.  Could anyone assist?

In Summary

• The computer sits on my desk, it is not near anything that would prevent ventilation.
• The computer is clean, and all fans and intake areas are free of dust/obstructions.
• The virus software is up to date and reflects no virii/malware.
• All software (including the OS) is up to date, including Windows 8.1, DirectX, and the NVIDIA graphics card drivers.
• There are no cords or other items obstructing the fans inside the computer.
• The graphics card is brand new, right out of the box, a week old (and it has happened now with two different graphics cards.
• GPU-Z sensor software shows that the GPU temperature is steady in the mid-to-high 80's and peaking at 90 degrees celsius before the computer shuts itself down.

Any assistance you can give would be much appreciated.  I am not able to afford a new computer now, and was hoping to save this one, since it's only a year and a half old.  Thanks!