I had the same problem. I had a Fram filter on at the time. I switched over to NAPA GOLD (made by WIX) problem gone! I even added a
oil cooler which everyone said would my problem worse. Trust me. Go to NAPA tomorrow by a NAPA GOLD filter #1515. Do the swap and post back your results.
One more thing. If you are running roller rockers make sure you put rstrictors in the heads...this got me a couple PSI. Thanks to George Anderson.