I am not saying this is your problem, but i went through almost the exact same thing about 2 years ago. I had a motor i did for my self. I had been driving it for awhile and all of a sudden it lost oil pressure. The same deal as yours. The gage would go to about 20 psi and that was it. I switched gages, changed oil and filter but i put a fram back on. I then switched to a different brand of filter (k&N) and the pressure came right back up. I have found that the fram filters get clogged with the break-in lube real easy also....
Just something to think about.