[stugiv]

Sounds like a problem I had on a similar vintage Mercuiser 454. Oil pressure was fine at startup & would slowly drop. Turned out it was the oil sender on the engine. They have diaphragms inside & if it has a tiny leak the oil slowly leaks to the side with lower pressure until they equalize. After shutting down, the oil will drain out & it will work again for a little while.

New senders are a pretty cheap & easy fix if that's the problem.