Here's the hardware:
1 x ASUS P6X58D Premium LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX Intel Motherboard
1 x SAPPHIRE 100281SR Radeon HD 5870 (Cypress XT) 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card w/ATI Eyefinity
1 x ZALMAN ZM750-HP 750W Continuous @ 45°C (Maximum Continuous Peak: 850W) ATX12V V2.2 / EPS12V V2.91 SLI Certified CrossFire Ready 80 PLUS Certified Modular Active PFC Modular LED Heatpipe-Cooled SLI P
1 x Intel Core i7-920 Bloomfield 2.66GHz LGA 1366 130W Quad-Core Processor BX80601920
1 x OCZ Gold 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Low Voltage Desktop Memory Model OCZ3G1600LV6GK
Boot Drive (SSD):
1 x Crucial RealSSD C300 CTFDDAC128MAG-1G1 2.5" 128GB SATA III MLC Internal Solid State Drive (SSD)
1 x Western Digital Caviar Black WD1002FAEX 1TB 7200 RPM SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive
1 x Western Digital Caviar Black WD1501FASS 1.5TB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive
Windows 7 Ultimate.
Ok, so the problem. The computer suddenly shuts off, especially when there's some load on the video card, such as playing a game (Deus Ex 3) or running a benchmark (Heaven or sometimes Windows Experience Index). Doubling the typical auto fan speed on the video card from about 22% to 45% definitely helps, although the temperature indicated by the Catalyst software shows it rarely going over 50c. However, I just got a replacement under warranty today (same model, different card), but it has not solved the problem. It just rebooted again during a Heaven benchmark, so I moved the card to the next slot on the mobo and the computer rebooted after about 5 minutes of Deus Ex.
If it does reboot, it will usually reboot again before Windows gets loaded up, and once it even rebooted like that while I was checking the BIOS. One reboot means I have to intercept it and shut it off for at least a few minutes before starting up again. It's been doing this for about a month or a little more.
If not the video card, I don't know what could be causing this. I'm planning to replace the memory (we tend to run a lot at once) as soon as I can figure out what would be best to get, but I don't expect that would be the problem. However, we did have a memory issue in the past (resolved with a previous question here), so I don't want to rule that out.
There's also a weird thing with the BIOS sometimes changing the boot order since we got the SSD. I don't suppose it's related, but I don't want to rule anything out without knowing.
Thanks in advance for your help.