maybe the monitor doesn't support the screen resolution or refresh rate the Mac is trying to use after it goes past the gray Apple screen.

have you tried resetting pram or nvram, have you tried to boot from a tiger install cd to see if the monitor will work that way.

BTW, what brand and model is the monitor.

i haven't ran into this issue before (i might be asking the wrong questions).

can it be a video card issue or driver issue. have you hooked another monitor up to it to see if it does the same thing. have you checked the end of the monitors connector to see if the the pins isn't bent.