Be sure your rocker nuts are in good condition. The stock ones cease to be lock-nuts after a lifetime of heat, and if they lose their tension, they will un-adjust themselves quickly.
Here's an easier way to do a running adjustment that avoids the stumble when tightening past zero lash. Loosen each one until it taps, then tighten it until it stops tapping, but no further. Then do the other 15 the same way. Then shut off the engine. Then tighten each nut one full turn. Or a half-turn, if that is your preference. (the factory recommends one full turn, but not everyone likes to use that figure)