From the sounds of it I would think it would be something with the pickup. Either something blocked, or a bad seal on the pickup tube. If your getting 50PSI then I would think the pump is working, but at lower RPM if there is a blockage on the pickup it would be hard to pull the oil up, but at higher RPM it does pick up more. Not sure if that is where the problem is, but it makes sense to me.
webracin