Start and Ignition Problems on 71 Demon

After further testing, spark at coil is weak and intermittent, cranking engine over gets a small spark at coil every 5 sec of cranking or so....any ideas? Tried changing the coil out, no change...and measured voltage with key in run, battery voltage at both sides of ballast resistor (12.25 VDC or so), and positive and neg side of coil show .682 VDC...measured it several times....not 6.82....I'm stumped...

Thanks,

Mark