Tuff 150 amp alternator

This can be "worse." First we need details, !!!DETAILS!!! on the system.

1...Just what is a "Tuff 150A alternator?" Who makes it and what does it use for a regulator

2...How EXACTLY is it wired? How large is the feed to the battery? How is the alternator grounded, and what is the path to the battery ground

3...How large is the charge wire and what exactly is that path?

Oscillation in the old days could be caused by a electro--mechanical regulator, which you do not have, I'm sure. It can also be caused by various wiring problems, such as poor connections, improper grounding, and TOO SMALL conductors (same idea as bad connections) which cause voltage drop in the system

AND KNOW THIS!!! 150A is nothing to sniffle at. You have a WELDER there. You have something that can burn that entire car to the ground.

It could be caused by electrical problems in the VR or alternator, but less likely.