Battery needle moves

What is your question?

Hello i have a 1994 ford mustang gt 5.0 and when i idle the car battery fluctuates back and fourth but when i drive its fine and the other day the battery needle went all the way down and i stalled out..Ive checked the alternator and its fine

What makes this problem better or worse?

When im idle at a stop sign

How long has your 1994 Ford Mustang had this problem?

Just bought the car