1997 502 mag mpi starboard side water pressure is fluctuating from 15 to 25 pounds at 3000 rpm when running. The port motor gauge holds steady at 25 pounds. I am going to change the sea water pump impellers because I think that may be the problem but looking for any other things I should check also in case that is not the problem. The impeller was changed 2 years ago. Could the gauge be bad?