if the lifters are suspect the .006 th would be a for sure cure 0 lash will have preload when the engine warms up 0 lash would work when the engine is warm. when I do 0 lash warm I make sure the push rod will still spin when I am done then I can be sure there is no preload 
the push rods can hang up on the edge of the lifters so check that out also 
Bob

Ken wrote 
How did you adjust your valves?  If you adjusted them per the Bentley Manual
(2 turns) this could be your problem.  Check your manifold pressure (vacuum)
and if it is low (should be around 12-14 at idle) then you could try backing
off your valve adjustment to zero lash to see if this helps