I had a boat that did exactly that...........oil pressure started to drop into the 6 pound mark at idle as time went on. Changed the sender and oil pressure went back up to 20 or pounds at idle. The suggestion to try a mechanical gauge first is a good one, even gauges can go bad.

You might also drop the pan and put in a new pump as Richard suggested. The relief spring can weaken with age or the pump can start going bad. We used to stretch the relief spring out in the old days to get higher oil pressure.

Don