That would make me think of a particular lifter having issues, or an adjuster getting loose, or some other wear issue. The loose adjuster ought to be obvious. A weak lifter sometime can be found by first running the engine for about 5 minutes to get the oil warm and the lifters pumped/filled up. Then stop engine, remove valve covers, and press down on the pushrod end of each rocker; any soft lifter found at that time is a possible suspect. These are just some suggestions of what to look for and methods.
