Thumping noise, oil pressure drops and goes back to 3

What is your question?

When I am stopped at a traffic light. Oil pressure drops to number 2 and feels like cars gonna cut off. Also does a thumping noise and car vibrates. Put car in netural and it runs fine. Put it back in gear starts thumping noise. Drives normallay. Just at a stop does the thumb. Checked all fluids and everything is fine. This is the second time this has happened. Second time it started after I put gas in the car then it started the thumping and oil pressure needle jumbs up and down. When got to my desanation. Put car in park and runs like a champ. Dont understand whats wrong with it. Any ideas before I take it in?