Did you, perchance, leave something in the motor when you had it apart?  Don't ask me why I suggest this...I did learn, the very very hard way, to count each shop towel you plug openings with, etc.  during a rebuild, and then re-count them after removing them all(?) as you re-assemble the motor.

  I found one was sucked into my Porsche motor and sitting in the pan..After the oil warmed up, it would migrate onto the pick-up screen and diminish the oil flow...On a very very off-chance, I fished around through the oil drain plug hole with a hook-bent piece of welding rod and got some scraps of paper on it....
  When I pulled the motor and removed the pan, there it was..a blue shop towel...Duh!
  Don Hanson