I bought a 71Mach1 with a 351W. It does not matter is cold or hot, or you step on gas or ease into it; it will feel like a miss. When it happens the tach drops as well. I initially maybe bad plug or two. I changed them all and none of them had any corrosion on them. Plug wires look new with no burn marks or inside wire showing. Carb and intake are newer and can not see anything out of place. Any ideas on what might be causing?