Charging issue

My wife's car is having an issue with power. A few weeks ago I noticed a drop in electrical power with the battery signal light on the dash. Over 2700 rpm's the light turned off but came back on at low rpm. I reached my destination and when I started the car upon leaving it was sluggish but fired. Like a bad alternator. But, the car was running fine and had no electrical power loss after that until yesterday evening. She drove it to the store in the evening, 3.1 miles. While driving every signal light on the dash was on, engine was very slow and unresponsive, and when she got to the destination it would not start due to low power from the battery. I came to her and gave it some charge via jump cables and she drove home with no issue. I later started the vehicle and ran every electrical drain I could with a voltage tester connected. No unusual drain. Since the alternatives are not cheap for these cars and they are known to have electrical issues I though that I would get a little advice before taking the alternator out for testing or replacement.