The valves have a base setting that uses shims behind the lifter and between the lifter and valve. Since you had a valve job performed it would be safe to assume that the valve faces were machined as well as the valve seats. Removing material from both surfaces will cause the valves to sit higher at the top, possibly impacting this base adjustment. You may want to start by having this checked then adjusted if required. I’m not suggesting this is the actual cause but given the history it may be worth investigating before devoting anymore time to rooting around.

In true MB form it will require the use of Special Tools.