what did you set valve clearance at?
manual says 0.38mm (0.0150in) (hot)
what oil are you running in engine?
manual says to use API service classification SJ or above
SAE 10W-30.
if you meet all those qualifications then i would check the oil delivery to the rocker arm assy and make sure it is getting enough oil up there.
if oil is ok there then remove rocker arm assy and check for wear on any of the rockers and valve stems and also cam.
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