I took my car to a local alternator repairman who specializes in rebuilding and servicing Bosch equipment. He says that my alternator shows an intermittent failure to produce sufficient voltage, and that it varies from a high of 12.9 v to a low of 11.8 v. A load test of the battery indicated that it is fully functional.

I am taking the alternator off tonight, and he says he can test and replace or rebuild any defective parts tomorrow.

Thanks to everyone who offered advice.

Ken