Yeah, tis a little annoying, I've got some friends who are having the same problem, one of them thought it might be the graphics card.
Not sure about getting a new power supply though, the one in the G5 might be closely tied to the other components for regulation, so you might have to be careful about replacing it.
Let us know how you get on .I'm thinking of moving to a G5 next year for AV work, so be nice to see if this is a problem in that area.