Check your ICU, Coil, Stater & Rectifier,
It sounds to me your problem might be the ICU.
If that goes, the fault could any or all of these;
ICU, Rectifier, Stater and/or Coil.
I've gone through 3 ICU's myself and they are expensive, this time I'm replacing all 4 of these parts...
Pretty sure that its the Rectifier on mine.
download the service manual and go to the troubleshooting section (I think its 1-18)
it lists symptoms and cures, then just follow the manual and you should be able to figure it out in no time