I know some fluctuation is normal since it is computer controlled. Sometimes those things will test good as will batteries but there still may be a problem. How old is your battery? If it is more than 4 years old I would replace that with a high quality part first and see what happens. We do replace alternators on those cars every so often in the 100k range. Another thing i see sometimes are corroded battery terminals, if they are bad enough that may cause an issue charging as well. Sounds like you have addressed that though. Chevrolet service consultant/ tech here. Typically if a regulator fails it would not stay within the parameters that you are seeing.
chris
