Two nVidia Cards crashing to desktop but ATI works?? Overheating?

I've been having issues with games crashing to the desktop for some time. I've ran diagnostics, tried swapping cards - kind of at a loss. Thinking about just buying a new card but a bit hesitant - figured I'd run it past everyone here to get some thoughts. I wouldn't say I'm a noob when it comes to hardware - but I'm definitely not a gaming hardware Geek. I have my A+ certification - I work as a tech - but I troubleshooting these type of issues is not my forte.

My System Specs
Intel DX58SO Mainboard
Corsair 850W Power Supply
Corsair Dominator 6GB 1600Mhz CL7 Memory 3x2G (Triple Channel setup)
Intel Core i7 930 2.8Ghz 8MB LGA1366
eVGA Ge-Force GTS250 1GB HDMI
Acer GD235HZ LCD Monitor
Using On-Board Sound (Realtek HD) via Analog plug.
80GB Intel X25-M SSD HDD
Cooler Master Cosmos S Full Tower Case - all fans operating - all filters clean - case has room to vent.
nVidia 3D Vision Kit
Steelseries WoW Mouse + Software
Razer Lycosa Keyboard.
Windows 7 Professional, 64-Bit
Symantec End-Point Security AV
Latest Non-Beta nVidia Drivers
Have updated all other drivers / firmware, and have all Windows Updates.
I am not overclocking, although the Intel Turbo Boost is enabled in BIOS. I haven't disabled it since I was told it is only used in single core apps and that it is a fairly stable implementation.

Crash History during Gaming
Using my GTS - I started noticing crashing to the desktop when I first started playing Civ 5. Then this summer it was doing it non-stop for DragonAge Origins. Up until that point - I figured it was the games.. there was a lot of forum activity around Crash-To-Desktops. A month ago the crashes in DragonAge Origins got really bad and was happening more and more frequently. I always noticed that my GPU fan would speed up .. hit max fan for a few minutes and then the game would crash. Last week I purchased TES Skyrim. Running on full settings, I played for about 2 hours and then the first crash occurred. It was a hardware lockup - I had to do a hard power off. I restarted system, went back in and the second crash occurred within 5 minutes. Third time I lowered the settings down to low - system was still warm - and it crashed within 10 minutes.

Troubleshooting Steps
I ran GPU-Z and noticed that my GPU temperature was peaking at 75 degrees .. and that was about when the crash would occur.
I was below 60 degrees on the motherboard temperature sensor. From my understanding, both these temperatures are normal for a system under load.

I went to the PSU requirements calculator and entered all my components in .. came out saying (if I recall correctlY) that my system required 450 watts .. 600 to be safe. So I am well above the requirement.

I pulled an XFX 8800GT out of my other computer and threw it in. Was able to play the game for about 6 hours on low settings until it crashed. It wasn't a hardware lockup however - it just dropped me to the desktop without error. I turned computer off for the night. Next morning, I turned the resolution from 1920x1080 to 1280x720. I played this way for two days without any crashes. The fans were not getting up to max. I threw my EVGA GTS 250 back in with the same settings and it was the same behaviour. At this point I was convinced that whatever the problem, it was only under higher load.

My friend came over one night to try out Skyrim - but he didn't want to play it at 720 .. so he brought his Radeon 5770 HD over and we installed it, along with the latest drivers. He cranked the settings to the absolute highest - and I've yet to have it crash. He left his card with me to use until this weekend - its been two or three days now of playing with no crashes or other issues.

After Thoughts
I haven't ran a memtest yet - mostly because I'm too lazy to burn a CD. I really don't think it would be memory however since the memory is on the mainboard's recommended memory list (which rules out incompatibility) and I would be experiencing crashes in all games - which I'm not. In the past 11 months I've played Oblivion, Torchlight, RUSE, Fallout 3, Fallout Vegas .. Didn't have any crashes with any of those games (I may have had a few but nothing that sticks out in my memory as being problematic). Also, Windows 7 has been rock solid.

Some people have hypothesized that its Windows 7 and DirectX 10 cards ... problem is that they heat up .. and when they go to scale back the GPU performance for temperature management, the Direct X stack crashes. I'm not familiar enough with the hypothesis to weigh in on its validity - although it sounds like as good of an explanation as any.

Right now - I cleared about $600 from my budget and am prepared to go upgrade my graphics card. Figured I'd stay away from EVGA and XFX and try out an ASUS this time. The ASUS Matrix GTX 580 has the DirectCII cooling - it seems like a card designed for overclocking - so if I don't overclock it - it should have good temperature resistance. Its also, of course, a newer DirectX 11 card.

I plan on running a 12-hour memtest on my computer before buying the card however - just to do my due diligence.

Anyone else have any other thoughts or suggestions?

Clean your gts 250 out with compressed air.

Card is clean. After a few weeks the ATI card started acting up (even outside the game). So I don't know.

I swapped my old card back in .. cranked the settings and it hasn't crashed in the past week - so I don't know. There had been a new BIOS update .. chipset update, and I used the nVidia Beta drivers .. so who knows maybe the whole issue is software based? Windows's Solution center has analyzed the crashes and came back with the solution that the Graphics Card Driver had stopped working .. so perhaps it was the earlier nVidia driver that was the isssue.