This same symptom happened to my 640. Drove me nuts.
As it turned out for me, the points had been modified to do away with the copper strip and the plastic square and screw, and the wire from coil to points was run through the hole in the distributor directly to the points, as has been described here at length. Well, on mine, the wire was getting flogged by the advance weights, which ripped off the insulation and shorted the coil to ground at inappropriate times. Sometimes a bright blue spark, sometimes no spark. Sometimes start, sometimes start and quit.
Good luck, just passing on a 'gotcha' that drove me nuts for a few weeks.