Sounds like the problem I had when I was building a Pentium 4 based mini ITX system. I was using power supply boards that have external AC adapters (like a laptop) and they were supposedly powerful enough to run the system, but as soon as the graphics drivers were installed or any load was put on the graphics chip the monitor would go into standby.
Using a more powerful standard power supply fixed the problem, so I ended up going with a slim 250W FlexATX PSU.
Before this thread, this is the only time I've ever heard of such a problem. Its good to know this isn't some kind of defect with the motherboard I'm using.