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
Thanks for the link Tom, very useful info.
I forgot to mention that there's no battery installed. The last one I bought didn't seem to work
since every power failure it didn't keep date and time so I decided to remove it.
Maybe what it needs (according to the link you sent) is a total CUDA reset?
Thanks a lot