Checked your timing gear?

he my 340 from about 2 yrs ago time would jump all over the place put a new comp cam chain in it same slop same thing with jumping around...took motor apart and re ring,bearing and cam, gasket and add a cnine ten. and no problem with the time now except a little slop in the dist. need to tighten the shift up a little in the drive...soon to come....i plan to use the ten. for all my new builds ....read on moparts that some of the big name (builders ) had a post and they would not run without one...8)