Keith - the only time I've had it happen, there was a set of points stuck in the V/R.. I "fixed" it with a good whack, evidently scared the crap out of that gremlin, never came back...
I believe it is the cutout relay in the V/R that would be sticking - this is the GEN connection and if it were stuck, it would provide DC to the same field winding that is supplied DC by the starter solenoid .. If you look at the wiring diagram
here, you'll see what I mean. If this happens, disconnect fairly quick, cause the wiring from the GEN terminal is much lighter than the big battery cable and you'll pretty quickly let all the smoke that's in that GEN wire connection escape..
BTW - that key switch could be putting power to the solenoid all the time...........