If the P ain't making the marks dance & if you are sure the chain is tight then turn the rotor slightly back & forth & see how much circumferential play (rotating distance) you have as 2 deg there (intergear slot/dist lower tang interface play) can make the marks vary in a 4 deg range on the dampener which is the "spark scatter" that you see with the light. Stock 400/stock intake/eddy 1406. what cam? stock also? with 16-18 initial what is your idle speed and are you on ported or manifold? Still sounds like a vac leak & you do need a strip kit but that is unrelated to the jumping. A stock 400 with the only swap from OE being the eddy 1406 should idle dead smooth at 500-600 RPM. holler back with more info as I may have missed something
