i have a 1970 mustang with a 2 barrel 302. motor is all stock. i have a terrible vbration between 900-1300 rpm. ive tried everything, ive got good compression, 165 in every cyl. no vacumn leak, changed plugs and wires, changed carb. installed petronics ing. and msd 6a. vibration gets worse after 30 miles of driving. doesnt vibrate while driving just idle. feels like something is out of balance. any ideas.