I have a 2001 mercruiser 5.0 L MPI I'm running mercruiser 25-w40 oil i noticed the other day my oil gauge dropped from 40 psi to around 20 psi at idle. While under load its at 42-50 psi but if i cut back on the throttle it immediately drops to 20 psi then works its way back up to around 40 .I had a marine mechanic put a mechanical gauge on it and he says everything is fine and that its in the normal range between 35-60 psi.My question is why all of a sudden has the gauge started acting up i already replaced the sending unit and I'm still having an issue. Any help would be greatly appreciated.