video lost a few seconds after boot

I'm trying to install an Inspiron 580 at a new location, with a connection to a monitor identical to the one it formerly used (Dell SE198WFP).    The system worked fine in its old location.    After I set it up again, it powers on and the monitor powers on.   It seems to boot normally, but after several seconds the monitor goes black.    I've tried booting in safe mode, same thing.   Used F12 to get into menu for diagnostics, same thing.   No matter what I do, I get a video display briefly, then a black screen.    I've tried connecting and reconnecting VGA cable and power cords.   I've verified that auto detect is working and setting the monitor to VGA.    Nothing connected to the computer except monitor, keyboard, and mouse.   The 580 has integrated graphs and no video card installed.    One possible clue:   during one attempt (not sure which), I got a brief message about power save, but it disappeared too quickly, and I wasn't able to reproduce it.    If the system IS somehow stuck in power save mode, how can I get it out when I can't use the display?    My next step is to take my other monitor and VGA cable there and try the system with that.    If that doesn't work, I'd appreciate any thoughts on other strategies, before I take it to a repair shop.   (I've worked inside a system in the past -- inserting a new card or hard drive -- but I don't feel comfortable enough to mess with connections, etc.