John:

I had a similar but not quite as extreme situation in my 1985 745t accompanied by an overcharging situation. I didn't diagnose it with a voltemeter, but the extreme changes I was seeing in the included in-dash meter were enough to tell me the general issues.

the overcharging sitaution was fixed by changing the voltage regulator, but I imagine that was fixed if your alternator is new or was rebuilt.

I posted about the vacilating alternator dial and was told by some of the more informed posters that it might be the harmonic balancer. Supposedly as the balancer goes it can effect the evenness of the charge from the alternator, etc., and can cause an issue like you are seeing. Additionally, have you changed out your belts to make sure you have new ones and that they are tensioned correctly? maybe your belt is slipping?

good luck,
Rick