First try a PMU reset See How to reset the SMU/PMU on a Power Mac Earlier G3, G4, G5 models...
Your internal PRAM battery may be weak or discharged. Suggest you replace the battery. Check this site for battery part numbers and sources. https://sites.google.com/site/macpram/mac-pram-nvram-cuda-pmu-battery-tutorial Mac PRAM, NVRAM, CUDA/PMU & Battery Tutorial