Fluctuating water pressure
 

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?