Little M block. Do you have internal restrictors in the block? Small restrictors and cheaper oil pump will create the pump to cavitate, open the internal pressure relief valve and bypass the oil back to the pan. If you had a clear oil pan you would more than likely see foam pouring out of your relief valve outlet on the pump. I don't believe 10550 pump is an anti-cavitation pump, I do not reccommend it on a block with main priority oiling blocks such as the Little M. You need a Melling 10552C which uses a 3/4" pick up tube, and has anti-cavitation properties.
Improper base circle cams and lifters will also cause this to happen. Sometimes the oil band around the lifter can hang belowthe lifter bore and bleed oil off rapidly and also make it cavitate.
you can easily check this by making sure you are getting adequate oil to the top end of the engine.