You can't test it, not for it's filtering ability or voltage stability, which is what this discussion is about.

Do not test it, replace it.
I responded to your PM, if you do not want to replace the battery, answer those questions I asked and MAYBE we can find a non-battery cause.

If it were me, that car would be getting a new battery today, then I'd be doing one of the two mods in THIS thread:
https://www.crossfireforum.org/forum...le-engine.html

That is the advice you are going to get here - cause so many of us have been thru this before. This is all new to you, but not to those of us who have been around these cars for ten years now.