I have a 77 K5 it has a large lunati cam , 35" tires 650 holley, the works . it ran great until last week . nothing out of ordinary happened. no mud holes or extremes. it just started idling really rough in gear but not in park or nuetral. it also runs rough and back fires while driving.I have already replaced- spark plugs and wires. all vacuum hoses, and cleaned carb and checked float adjustments. it acts like its out of time but nothing has changed in the 4 years I have had it. The rotor button looks good and coil is still firing.could the distributer just go bad?.any input would be appreciated.