I would agree it is most likely lifter pump down. It might be a tight valve guide, but with the time on the engine not likely. I think I would rather go with a better rocker set up, and the traditional ball and cup type push rod. Paying attention to pushrod length. The further the adjuster extends past the rocker arm, the arc of movement, will have a greater tendency to kick out the push rod. The fit of the pushrod in the rocker is critical as well, no mater which type of push rod you use. I never use the type of rocker arm you show because of the issues I mentioned. Just me never liked the look of them.