Troubleshooting Start-up Issues

My XPS720 works fairly well apart from a little disturbing thing. It does not always start.. It goes past post but stops dead just before the point I expect the DVI symbol to appear top left and the Vista orb to appear. Just stops black. I get nothing in any logs and no error message. I have updated all my drivers, GPU several times. No difference. My RAM is fine. Anyone able to suggest how troubleshoot a problem like this one?