I have had this exact same problem for several months now on the MB899 motherboard. I can't count the number of times I went into VGA Mode, uninstalled drivers, and reinstalled. But, I did find a way to correct the no video display without a driver reinstall, though it does not really explain why the video dropping happens.
The Intel graphics chipset for 945 has a built-in set of hot keys for display switching and screen rotation. If you plug in your VGA monitor and have no video, use the combination of [CTRL] + [ALT] + F1 to force the graphics to switch to the VGA port. If you use the RCA output for video, use the same combination, except press F2.
Like I said, this doesn't explain why the video drops, but hopefully it will save everyone a lot of time like it did me. It does say in the MB899 manual that if you plug in a VGA monitor while the machine is running you must use the hotkeys to bring video up on the display.
If anyone finds a concrete explanation for this and how to permanently remedy it, please let me know.