One of the first things to check is the
oil pickup tube. Often the tube will crack near a weld or on the "foot" were the screen is.
I onced experienced the very thing you speak of, it was a hairline crack in the weld itself where the foot was welded on the tube. I never suspected this. As the engine consumed a minor amount of
oil due to bad valve guides the crack was uncovered intemitantly
causing the
oil pressure to fluctuate @ 40-50 lbs (15 lb swing) above 3500 rpm. Try incorporating an additional brace on the pickup tube anchored either in the windage tray or a main cap
bolt.
This was a problem in the early 289 race cars too.
Good luck .........Rick
Part of this was due to oil drain down