adjusting at the valve stem right? When I adjust the valves on my vanguard twin. I have to do it about three times. Adjust start up, watch lifters, shut down adjust again cause the gap is too big. I suggest you inspect for wear and bent rods, everything loose good then I would adjust, start up and verify you still have the gap you adjusted it to.

I have also noticed even though I got .004 gap by the time I tighten down the rocker adjustment nut the gap has changed. have to do it couple times to get it close. I know I got it close when .006 won't go, .005 is very tight, and .004 has slight drag. I also notice that it's hard to get the piston up and in the right adjustment spot if I don't remove the plugs and stick a dowl in the spark plug hole and watch the movement.