Probably the pickup/sender unit. I had an erratic old pressure reading once in the middle of a long trip and, with each bounce on the dial, the pressure reading dropped a little. Pulled into a Dodge/Chrysler dealership in Idaho Falls. Mechanic said immediately "oil pumps in these engines are built really tough and never fail". He checked and it was the sender. Replacement was about $100. No problems since. Or, as Bruceper says, maybe just low oil level.
1995 Dodge 350 Pleasure Way, 318 cid engine.