If you get to the point of taking off the rocker arm assembly: check to see if the mating surface on the rocker arms are warn. If they are, a quick fix might be to grind/polish those mating surfaces.
Re-install rocker arm assembly
cold-set the gaps
start and warm up the engine
Turn off the engine after warm-up
Set the valve lash - if you do have a technique for adjusting the warm valve-lash, ask.
People here have a few good and easy techniques.
Even if this does not quickly/easily solve the problem, you will at least have your rocker arms ready to go, after moving on to deeper diagnoses.
