Ed,
  Have you checked to see if the radiator core is clean? The water has to be able to flow from the bottom tank to the top (or is it reversed) at any rate, it may be worth giving the radiator a flush. If anyone has ever used any of the old radiator stop-leak, some of which has bits of cloth or something of the sort, it could be plugged with that stuff. The old fella I used to work for used that stuff and really made a mess of the old irrigation pump radiator.......just a thought.
                 Cal