You can try and adjust the rockers . But I think you may have a worn cam lobe . First take a long 3/8 extension and put it by your ear and by the rocker arm nuts you will hear the bad one . You can try to tighten it down 1/2 turn and see if it starts to quit down a bit . If it does remove that rocker and check if its cracked . If its ok reinstall it then adjust it . How old is that camshaft ? It may be time for a cam change .