You probably have the spark plug wires on wrong on the left side of the engine---no 7 and 8 need to be separated to eliminate crossfire---the wires generally ( from factoy) are 7, 5, 6, 8---and after many tune ups they get put bac 5,6,7,8 which has 7 on 5, 5 on 6, 6 on 7 which creates vibration, rough running engine just as you described

Jerry