G5 has these hardware issues:
-- bad capacitors on logic board
-- bad capacitors in power supply
-- badly soldered video chip
-- "There were several Mac models with flat panel displays that had bad displays.
Most had serial numbers beginning with "W8"---the code for one specific factory.
This affected PowerBooks as well. If that's the case, the rest of the computer
is probably fine." by Allan Jones in
Google: g5 capacitor replacement kit
More info on capacitor replacement kits