Hi, I have a 89 Rinker with a 4.3 mercruiser in it and the oil pressure is great till I get around 3500 rpm then the gauge jumps around a little then goes down to zero. when I slow down it jumps back up on the gauge. I never hear any knocking in the engine when it goes down but don't keep it going that fast that long. I'm just wondering if it could be the sending unit or something much more expensive internally. Thanks