drop the oil pan and check out the oil pump and screen...ensure it isn't clogged and as Blazin said, check for shavings.
while the pan is off, check out the rod bearings and main bearings. Put pressure on the rods and see if any play.
have someone turn the crank at the pulley w/ a 5/8 inch socket. Watch in from down below. Try to get a flashlight and check up at the pistons and all bearings.
after a good visual check of bearings and applying pressure and if no problems, put a new pump on it and button it back up and see if that solves the problem.