You said that the rockers are loosening up after a fresh rebuild. How far are you screwing the adjusting nut down when you adjust the valves, and are the lifters solid or hydraulic?
If you're running hydraulic lifters, the adjustment is one full turn after the rocker stops making noise when you're tightening the nut with the engine running. With solids, there should be an adjustment that you set with a feeler gauge. If you're doing all that and the rocker nuts are still loosening up, refer to the posts above. I can recommend the poly locks. I've used them before and they work very well, but are pricier than the rocker kits at Auto Zone and other parts stores.
