Hi

You could have one of two things happening.

First: Lifter bleeding down

Second: Camshaft wear/rocker wear

You'll want to start with a complete visual inspection of the valve train. Remove the oiling tube to get a good look at the camshaft.

You may need to remove the cam tower assemblies to completely inspect the rocker surface.

CAUTION: Shims located on top of the valves, work with care so these do not get lost (they do fit in the oil drain holes located on the exhaust side of the cylinder head)

Let us know what you find!