Monitor blacks out during startup


Have a minimac. My monitor blacks out on the moment it should display the finder. It used to work but after doing a system update which crashed this happened. I re-installed the system but that didn't work. When starting from the system CD the display works fine. Any ideas?
Sounds like dodgy video drivers or something similar. I thought hardware problem but if it doesn't do it when using the system disc it couldn't be that either. Have you tried reinstalling the system after doing a full erase?
I connected a expensive monitor to the mini mac and the problem vanished...
After that i tried updating my system and while updating the monitor turns black again. Restarting won't help. Re-installing system won't help (not full erase though).
Can anyone help me out with this problem?
Perhaps the resolution set in the Displays pane of the System Preferences is set to a resolution that the monitor doesn't support.

Try hooking back up the monitor that works, set the resolution to something you know the other monitor can handle, then shut down and switch...