G5 ppc monitor issues

My monitor one day went to black screen, after trying to turn it back on it flashed on for half a second then back to black. I restarted, stayed black. 20 min later I restarted again, monitor worked fine. Continually cycles like this now. So I got a new monitor, same issue. I use a ppc g5, not sure what my graphics card is but I do use a VGA-DVI adapter with the monitors I have And it has two ports. Help! Graphics card? Adapter?