Computer Shuts Down Automatically. HELP!!!

Hi,

I just purchased a new system and been having problems since I purchased it.

System Spec:

- AMD PHENOM II Quad Core 940 Black Edition CPU, 3.0GHz (125W), 8MB Cache, Socket AM2+
- ASUS M3N-HT Deluxe MP Motherboard
- Thermaltake Qfan 850 Watt PSU
- G SKill PI Series 2 x 4GB Kit DDR2 1066
- Seagate SATA 1.5TB
- Asus Cooler Triton 78 Universal
- 2 x Inno3D PCI 512MB 9800GTX+ x 2 SLI
- Antec P182 Performance One Case

The symptoms are that the computer shuts down automatically at random intervals and when I go to restart it says no signals to monitors but system is running. I am also running a 3 monitor setup.

Have taken it back to where I purchased it from , they already have replaced the motherboard and same problem occurs. They have tested every part seperately and say everythings fine so they suspect its the m/b again. They have tested temperatures and all ok.

Don't know what to do. Would anyone know how to fix this? Could it be PSU or heating issues?

Would really appreciate your assistance.