I'm not a big advocate of just replacing parts to attempt a fix, but I think I tend to agree with the above advice in this matter.
My Delco OEM battery died at about 4 years and was replaced. The alternator had no problems and is original.
Bet it'* the battery, and I'd start by replacing that. As to the alternator, pick up a cheap $10 Chinese voltage tester at Harbor Frieght and see where you're at there. Intermittent faults in alternators are, in my experience, rare. If they fail, it'* all at once and for good, and then the battery runs down.
Any competent battery shop can stress-test the battery in a few minutes and tell you if it'* failing.
IOf course, if the battery passes the stress test and the alternator is alternating , it'* time to look elsewhere.
1999 SLE K engine