Old trick..........disassemble rocker shafts.....rotate shafts 180 degrees and reinstall. May have to drill/tap holes for lock screws.

While apart, examine shafts for grooving and egg shape. Do same with rocker inner bores.

While you are fiddling with it, get yerself a tube of machinist's blue(prussian blue).....or use ink from ball point pen shaft....Apply thin coat of blue/ink to rocker screw tips and cam lobes....roll engine over at least 2 full revs...then examine valve stem tips and rocker follower pads to see where blue transferred....You may discover that cam lobes are unevenly worn. And it will also indicate if screw tips travel to stem edge when actuating valves.