You said you installed a dual 1 Ghz card, you might all ready be aware that the heat sink will only work if the pad on the heat sink that goes over the CPU's is centered and has a thin coat of thermal grease, If it's not centered (it can be mounted 2 ways only one is correct), or missing the silicone grease the CPU's will overheat after a short time and shut down, on the higher speed G4's correct mounting of the heat sink is critical.