I once had this going on, turned out to be a crack that had developed in the foot of the
oil pickup tube where it was welded to to the strainer. At idle or low speed pressure was normal as the crack was (IMO) totally immersed in
oil. When RPM's rose such as steady cruise and
oil was being removed from the pan at a faster rate than draining back, the level would drop exposing the crack. Not a good situation, glad I caught it early. That was several years ago, and I have had no negative long term effects.