Voltage Gauge Bouncing

2003 Ford Ranger XLT 57,000 Miles. Back in June I had to replace my battery when I came back from out of town and could not get home from a friends house. Now when I have my headlights, running lights, and my heater on my battery gauge drops to low. At times when I am driving it will move back to the center while I am pressing on the gas petal and when I slow down and lift off the gas it drops again. I have not had this problem in the past and I have had the alternator checked and the battery checked, along with the connectors. However I am still told there is no issue with my truck. Is this normal and nothing to worry about or is there another underlying problem that may be causing this? Any help would be great!