The problem relates to the Video cards. If u turn off SLI you will find that the system becomes stable. I believe that there is a timing problem with the motherboard and the cards. If you bug dell enough they will replace your unit or so I've read. I personally gave up after the support people were at my house replacing NVIDIA 280 several times to no avail.
What I do is Power on, wait till the fan starts to slow. Hold in the power switch and turn it off. Wait 5 seconds then turn it back on. That works for me.