I don't have this vehicle, but our 2007 Chevy Uplander occasionally (once or twice every few months)does this. Normally, our voltage output cycles between 13.4 and 15.2 volts. It has never had any other electrical issue (or symptom of one), but I'm posting because your Envoy may have current sensors around the power cable from the alternatorlike ours (which are used to adjust alternator output) and perhaps that could be a cause of what you're experiencing. BTW, we have about 183000 miles on our van. I hope this either helps you get to the bottom of the problem or gives you peace of mind (if your symptoms are as infrequent as ours).