I own a 69 Mustang convertible, with a 302 2bbl.
Every few years, I start the car, and the engine will turn (maybe) a quarter of a turn, and then the car will go electrically dead.  

If I disconnect the voltage regulator, I at least get my interior power back (courtesy lights work), but the car won't turn over.  

Initially, I thought it was a problem with the voltage regulator, and I've replaced it twice.  The last time, I thought it may have been a battery problem, but the battery checked out OK.  (although after a thorough charging, the car then started)

Any ideas where my problem may lie?  Alternator not charging the battery properly?

-tjh  

Well, I see you mentioned that you start the car every few years. As with anything that sits without being used it always tends to find the weakest link in the system and break down. I can also say that I had a similar problem on another car where the battery showed a full charge but the car would attempt to start and then give out. Before I started tearing everything apart I changed the battery and that was the problem. An old battery that seemed to be at 100% charge but would not start the car. Replace the battery first then go from there.