"at rest" the field circuit ain't grounded (you have no continuity to the alt case) but if it is full fielding with the blue wire field lead plugged in & nothing on the other male alt field terminal then the field circuit has to be grounding/full fielding while it is "running". Am I seeing this correct per your description? If so I would take the alt in to O'reillys for a free check & let them spin it & tell them how it is acting so they can not only check max full fielded output (their std procedure which ain't the issue here) but see if they can get it to FF at a lower RPM (like it is doing now). We'll get it