This sounds like either a shorting problem or lack of voltage. By your readings you are getting adequate voltage, so it may be an intermittant problem. They are never fun to find.

Maybe the alternator brushes are worn and even though it is showing good voltage, the amperage is too low. The brushes are in a holder that comes out of the back of the alternator. You can then unsolder the old brushes and put new ones in.

__________________
Doug

1987 300TD x 3
2005 E320CDI