oldtimer new to rodding..i have a 350 sbchevy that is running ruff off and on. 270h compcam, headers, msd, 600 edelbrock on performer. vacuum is reading 15lbs at best, drops 11lbs when it gets ruff. when i touch the back of the screw driver to the exhaust tubes, #3,5,2,8 tubes are cool and the other four are hot. i put the timing light to the cool lead wires and they were firing. where should i start to trouble shoot?