My monitor shows no display (black screen) for a long time (15 minutes or more) after the computer boots up. Then all of a sudden it will start to display the screen. Is this a monitor problem or video card. I have an NVIDIA Geforce 7900 GS video card.
try the monitor on another computer if the problem wasn't there then it's your GPU, also try another monitor on your computer to check if it's truly a GPU problem
but probably yes it's a GPU problem