Give priority to advise posted here by others more knowledgeable than I am particularly as regards electricity and diagnostics, but I will share my experience with you. I had similar problems with a 760 turbo (1986). Diagnosis from specialized shop was bad alternator to be rebuilt, come tomorrow. That night I loosed the bolt that holds the ground (negative) cable from the battery to engine block and sprayed plenty of dry wire fluid on the connection and tightened the bolt. That solved the problem. In view of my new found prowess, I became ambitious and cleaned and sprayed that ground and both battery terminal connections. I also sprayed some WD40 inside alternator, particularly around brushes. Since then, the system is charging perfectly and better than it ever did during the many years I have had the car. Needless to say, I did not have the alternator rebuilt. I did change the engine wiring harness, but that was weeks before all of this happened.