I believe that there are cross over tubes that feed the lifters oil (like the M119 Engine) they are made of plastic and tend to crack or break down over time. thus giving less or no oil pressure to the lifters.
now you may just get away with replacing the cross over tubes.. but if its been happening for a while then you may need the lifters as well.. cam too maybe. depends on the damage.
my guess is just the cross overs and the lifter.

anyone confirm the cross over tubes on the M104 AMG engine?