a few years ago i recieved a engine that did the same thing, great oil pressure at idle and mid, but as soon as you stab it the oil pressure dropped, let it idle and the pressure came back. Tore it apart and what had happened was the builder put .020 rod bearings on a .010 rod journal..... funny huh? Cleaned it and put the right rod bearings in it and it was fixed. Hopefully this is not your problem!