Oil pressure flucuation

1972 chevelle with a blown 427, mileage unknown. Problem is lose of oil pressure above 3500rpms? pressure at idle 50psi cold 45psi warm. oil pressure is fine but starts to flucuate at about 3500 rpms starts to fall from 45psi to 20psi as low as 10psi? ussually can let off throttle and it will come back up? but sometimes it will drop and raise back on its own? It does this quickly jerking the oil press needle? Hooked another o/p gauge and it did the same thing? has auto meter good gauge with steel braided feed line. Can there be air in line? possibly sucking sump dry? Has a 6 qt milodon rear sump pan. even added extra qt of oil to see if sump was going dry. could pickup tube have come off? Could spring inside pump be bad & blleed off pressure to soon? Any help[ would be apprieciated thanks Darren