It sounds to me like the person who tested your alternator did not check for bad diodes; and otherwise did not know how to properly test it, but there are too many possibilities here for it to be worthwhile to guess at a cause; without being able to physically inspect and test the car. You need to find a good auto electrical and diagnostic specialist. There should be a listing under that category, or one like it, in the yellow pages of your local telephone directory. You could also log on to http://www.whitepages.com and go to the "Find a business" section; and type "Automobile electrical service" or "Automobile diagnostic service", along with your city and state. That should bring up a list of all the shops in your area which do that kind of work. If there are too few listings in your town; it might be necessary to try a larger nearby town.
If you find several shops and need to decide between them; pull up more information about the shops by clicking on the 'more info' button, and see if some of them list "diagnostic work" or "electrical repairs" as one of their specialties. That would be a big advantage in my eyes. Another advantage is a shop that has ASE certified mechanics. And another advantage is a shop that has been in business at the same location for many years, and has testimonials from many satisfied customers. An additional advantage is a shop that has been given high ratings in consumer surveys.