Help diagnosing a Overheating Problem

Forgive me if this is the wrong section of the forum to be posting this issue, I am new to posting in tom's hardware's forums but have found the advice indispensable in solving previous problems. But with this new issue I am surely stumped.

As the title suggest, I believe I am having an overheating problem with one component in my computer... problem is I don't know which part or what I should really do about it. The problem started a few months back when I was rendering an animation with a 3d modeling program called Blender. Because it takes so long to render the animations, I leave my comp on over-night so it will be finished in the morning. So time over the night however, the computer froze and it required the power button to restart the computer. This problem also happened when playing Battlefield 3, I would be able to play for about an hour, then the screen would freeze and the audio for that instant would loop, again requiring the reset button. The only remedy I have found to prevent this over heating is opening the case and putting a fan next to it.

Clearly this is not a very good idea, as more dust and dirt can get into the computer and shorten the life of the components, but currently I don't know what else to do. With the summer already in full swing, the environmental heat is going to play a big roll in this little drama (about 85 f at the hottest). This problem has me stumped and any insight would be extremely helpful! Thanks for your assistance!

Computer Specs:
Intel Core 2 Quad 8400 (at stock speed and stock heatsink)
4 x 2 gb of 800MHz DDR2 RAM
PNY GeForce GTX 560 ti
EVGA nForce 750i SLI Motherboard
Corsair TX750 PSU
Western Digital 500 GB Hard drive

Fan Set-up:
3 x 80 mm (One on side of case blowing in, on in front of case blowing in, one on top blowing out)
1 x 120 mm (Back blowing in)
Stock Intel Heatsink

Temperatures:
Rendering with case closed (a few second before crash):
GPU: 87 c
CPU: ~50 c
Hard Drive: 53 c
Ambient System: 57 c

Rendering with fan (no crash):
GPU: >71 c
CPU:>45 c
Hard Drive: 43 c
Ambient System:40 c

Stress Test w/ case closed (crashed after 20 min Prime95 + FurMark at default settings):
GPU: 94 c
CPU: 71.5 c
Hard Drive: 50 c
Ambient System: 57 c


I don't know what information you might need so I thought I would post most of it. If you need more info, please don't hesitate to ask. Again, thanks for you help with this problem!

Wingman_mn