1989 Tiara 3600 Open Oil Pressure Problem

[tjbtiara]

Mercruiser 454's with 680 hours, burn no oil and run great. Both oil pressure gauges have typically read between 40 and 60 psi at idle and under power. Occasionally the starboard gauge would rapidly swing or twitch back and forth then settle back into the 40 to 60 psi range. I typically cruise at about 3000 RPM's. Recently, after about an hour of cruising the port gauge started to rapidly swing between 20 and 40 PSI and the alarm light would flash. When I eased off the throttle to about 2800 RPM the gauge settled down to about 35 to 40 PSI. Soon after accelerating back to 3000 RPM the gauge would start rapidly swing between 20 and 40 PSI again, repeating the problem. I changed the port oil pressure sending unit and the gauge read a steady 40 PSI at idle. However, after about 30 minutes at 3000 RPM the gauge again started to swing and the problem repeated as described above. As soon as I exceed 2800 RPM the gauge would rapidly swing and alarm light would go off. I believe the oil pressure gauges are original, VDO. Both engines run at about 145 degrees according to the temperature gauges and the temperature does not change at all when the problem occurs. Any suggestions you would have as to my next steps to trouble shooting the problem would be much appreciated.