That jump in oil pressure at high rpm would be caused by oil flow from the pump exceeding the rate at which excess oil flow can be bled off by the blow-off valve. This can be dangerous because a gear pump is a positive displacement pump, meaning it will always move the same volume of liquid despite the pressure on the discharge side of the pump. As pressure builds, it will become harder to turn the pump and, eventually, something will have to give - probably the pump shaft or the pump gears- causing complete pump failure. Do you have a high volume pump? I'm not sure of the design of the blow-off, but if it's a spring and ball type, maybe you can go in with a drill and open up the passageway. Just don't drill the ball seat or the spring seat. On a old style small block, maximum pressure was 35psi. It is probably higher in the newer style engines.