most on car testers dont test amerage output, or they estimate it. a good load tester like an old vat 40 would do the trick. it might be cutting out under heavy load, possible bad internal parts that randomly fail, maybe even a short somehwere in the car that sometimes makes contact. theres really a ton of things it 'could' be. plug wires shouldn't cause this unless they are routed wrong and are very worn and are causing interference, but that would normally be all the time. i saw that on a suburban once. customer put on new wires and routed one right in line with a wire going to the cluster which caused the needles to twitch slightly.
what brand alternator?
also check alternator B+ wire. might be some green cancer in there
just remembered, we had almost the exact same problem on my moms minivan, 2000 windstar. random battery light. 2 advance auto alternators later and we threw on a motorcraft reman. problem gone.
