i had the same prob with my 600 efi, at idel mine made 12V but when you started to rev it up the Volts would drop and the fuel pump would loose power and then i would loose fuel pressure, put a gauge on it and watch your fuel pressure, if when you rev it up then the fuel pressure drops it eather your regulator or pump/power. u need a multimeter to test the stator to see what happens. when i took mine apart to take the coil out to have it rebuilt i noticed that 2 of the 12 coils on the stator where rusty, i figured what do i got to loose but an hour so i sanded them shinny again and put back in, it works great now, so if your stator is what you eliminated problems to, check that before buying. dont buy a cat one there 655 big ones, out of denis kirk there 320
if all the fuel/power checks out then go to your TPS, my 900 efi had that prob
i took my pump out and put it to a motor cycle battery and made the regulator work with hoses and clamps, it should hold till around 37-40. if that all works, check wires if not them then its prob your stator