Here's what i'd do in this order:
Check the battery to engine block negative cable, make sure both ends are clean and tight
The voltage regular ground from the factory was crappy at best. Run a ground wire from battery- to the voltage regulator body. Scuff up the body at a mounting bolt and mount it there, or add a little screw to the side of the regulator body and add the negative wire there.
Check the FLD wires at the alternator and make sure they're clean/tight, and trace them through the wiring harness if needed to check for any breaks/damage.
Check the bulkhead connector as well as the connection quality at the ammeter gauge behind the cluster.
Lastly, it could be a problem with the battery. Take the battery off, charge it to full, and reinstall it. If the same symptom occurs try another battery if you have a spare. It could also be a faulty alternator. I'd check all of the above first.
