Lifter rotation problem. Dirt in the lifter bores/galleys, scoring, wrong lube on lifter walls ( previously stated ). A good test: With the side of the lifters and lifter bores lubed with light oil, they should slide down the lifter bores with only their own weight. If you can't acheive this, you might want to rent a broach and broach the lifter bores - I think hughes rents them.