9 volts is WAAAAY too high for a 6V charging system to be producing, whether it's an alternator or a generator. The max voltage against a fully-charged battery needs to be about 7.5 Volts.
Voltage is ony part of the equation- - - -you need to find someone with a high-reading ammeter capable of showing the total current capability. Against a low battery, or with a lot of accessories in operation, the alternator should be able to put out its maximum rated capacity, 50, 60, 70 amps, etc. That's tested by putting a heavy load on the battery terminals with a carbon pile load resistor with the engine at a fast idle, and monitoring the output of the alternator at the main output lead. (the big one at the back of the alternator). If there's a bad diode or two, the voltage will be normal against a fully-charged battery, but there will be less than 1/3 of the rated current capacity available.
The symptoms you're describing are characteristic of diode failure which is knocking out two of the three phases of the stator, and partially blocking the output of the third phase.
Jerry
