You shouldn't be stalled that badly, I would have expected stall to limit you to about 10A not 3.
I don't like the fact that you say that it goes up to 3A and falls back to 1/2A.
Can you find another battery and try it at 24v as a temporary measure.
Also if you can easily get at the rectifier try it single phase by removing one ac lead at a time. It may be worth trying each one at a time, I have some reason to suspect you have a rectifier fault and the effect of each phase removed should be the same. If there is a rectifier fault then the effect will be different as you remove a phase..
If this is only stall , it will run perfectly well into 24V at winds above 10 mph and you should see at least 12A at 20 mph.
With it on 12v and single phased you should not stall but the efficiency will be lower. If the thing works much better with one phase removed than the other 2 then suspect a shorted diode in the rectifier.
Your alternator should cut in at 12v at about 160 rpm, a bit on the low side for an 8' 6" prop but not seriously so for a 12v system.
I think you said you disconnected the ac leads from the rectifier and it speeded up, try leaving the ac leads connected and disconnect the dc and see if it runs as fast and if you have a clip on ammeter, check for current in the ac leads. If there is current it again points to the rectifier.
Others have suggested a resistor in the dc line, you can also try this and it will help if it is stall. You will need something less than 1/2 ohm and you are probably more likely to find a long piece of cable or a short piece of steel fence wire than a dedicated resistor for a quick test. Cheapie ww resistors from mickey mouse radio supplies will fry.
If you can eliminate the rectifier as a source of trouble and prove that it works well into 24V then open the magnet rotor gaps to 3/8" for a first try.
If the rectifier is ok and it doesn't perform into 24v then you have the other thing that I suspect, turbulent and useless wind, I hope not there is no cure for that without very tall towers.
Good luck.
Flux