I have a 1973 Hurst/Olds,I have been recently experiencing a sort of surging.When the car is off,my stereo plays fine,when engine is started then my stereo starts to pulse(music cutting in and out)when I put my digy volt meter on my batt. I get a massive fluxtuation readout.I also get it on my starter and fuse block.(fluxtuation)
It sounds like a voltage regulator that’s not regulating the voltage.
A voltage regulator is $20 at Auto Zone auto parts. Check your repair manual to determine if you can change it without using special tools on the alternator. If special tools are needed, you’ll have to change the alternator, or, buy those tools.
Many cars of that vintage had external voltage regulators using a mechanical means to perform the regulation. It is common for people to eliminate them and install a more modern alternator with an integrated, electronic regulator. The first step is to determine what you have.