91 Civic oil pressure problem

I have a 91 civic with a 1.5 enigne with 257K on it. My problem is this. About a year ago I had problem with the oil light staying on after the engine was started but would go out in about 3-4 seconds. So I installed an oil guage after which I noted that the oil pressure was 70-75 psi when the car was started and settled out around 60 psi when warm, then the pressure would slowly drop off to about 20 psi. I replace the oil pump relief valve and spring from honda. Problem when away till a few days ago. Now the oil pressure is good when I first start the car but when you drive the pressure drops to 10-15 psi. The pressure slowly drops now when you rev the engine. I am really looking at getting a second opinion since it is a big job. Any advice would be helpful.