Just because the engine cranks and runs does not mean you have quality grounds and connections. More modern , computer controlled engine management systems run a ground to each sensor. A few 10ths of 1 volt do make a difference.
Connections need to be clean , tight , and mechanically sound. Wire splices and terminations need to done right. The toothed lock washers cut into the metal , paint is not a conductor. Silicone grease they sell at the auto parts for light bulbs is a good addition to inhibit corrosion in the future.
I confess that I even ran a ground wire from the grounding stud on the altenator to the panel the voltage regulator was mounted on. Remember I now have the internal regulator style alternator but ground is still vital.
I am a retire electrician . One motto is , " If you can't fix it with a hammer, it must be elctrical."
If electrical is a mystery there are good books on the subject. I have several books by Tony Candella , on basic automotive wiring. Purchased on Amazon.
There are 2 volumes and a EFI conversion book.