relay powered voltage regulator ?

With the car running to at fast idle, make this check first with all loads turned off, and again with lights, heater, whatever you can turn on

Stab one probe of your meter directly onto the VR mounting flange. Make sure to stab through paint, chrome, rust, etc. Stick the remaining probe into the battery NEG post. You are hoping for a very VERY low reading, the lower the better, zero is perfect.

Take the reading, and watch and see if that reading also pulses

This is checking the ground condition between VR and battery

Also work the VR connector in/ out several times to scrub the terminals clean and feel for tightness. Remove and inspect the VR and connector terminals for corrosion, etc.

It could be a flakey VR but that would be rare