You've probably got a set of cutout relay points sticking, which allows the battery to discharge through the generator armature with the engine stopped. A 12 volt system usually takes out both the generator and regulator when this happens. Sometimes a 6 volt generator survives the experience.
Take the top off the regulator and look for the set of points directly behind the Bat. lead. They should be open with the engine stopped, and they should close as soon as it starts. If they're stuck closed, the battery will discharge itself through the generator. Also look for discolored or melted wire insulation at both the BAT and ARM terminals, all the way to the generator. It's possible you'll also find at least a couple of burned commutator bars on the armature, where the discharge happened with the generator stopped. That's a worst-case scenario.
If the cutout relay points are sticking intermittently, sometimes it's possible to polish them gently with very fine-grit sandpaper, like 600 grit or so, and do a thorough cleaning with lacquer thinner or alcohol, followed by burnishing with clean file card stock. The points open and close with a slight wiping action, so definitely DON"T use a point file. The grooves you leave with a file will guarantee they'll stick. Good luck!
Jerry
