Rick
Your going to have to do some voltage checks at various points in the system with a volt meter .
But if I had to guess I would start at the alternator checking the voltage at the output terminal and compare it to the batteries and the voltage Sense terminal at the alternator looking for a volt drop .
Second check the voltage Sense terminal at the alternator this is the voltage the regulator uses to control alternator output
If you have a battery isolator with Sense terminal wired to it the problem may be in the isolator to verify this remove the sense wire from the isolator terminal and connect it to one of the battery terminals on the isolator and see if the voltage still bounces if it does the problem is likely in the voltage regulator in the alternator.
Hope this helps
Eric
