Oil pressure problem

About six months ago, I bought a 77 chevy c20 from someone on the side of the road. Ive driven it about 4500 miles since then, but I dont know the milage of the truck. The oil pressure has been reading around 20 at an idle and 35 to 40, while driving( new mechanical gauge.) Got up this morning to start the truck (outside temperture 25, with an over night drop of 20 degrees from day before.)Started driving down the road and as long as I was accelerating held about 20 to 25, when at idle it went to 0. Parked it immediatley, heard little knocking when at 0. Used motor flush and new oil change 10w30 kn filter. Started it with 40 to 45 at start but after it was back to 15-20 and 20 -25 maybe thirty. Thinking about changing pump and screen. Any advice would be greatly appreciated. Thanks