alternator problems?
 

About a two weeks ago I was cleaning my car and had the radio on for about two hours. Every since then I've had problems. The next couple days after, when idling my lights would get really dim and the volts would jump up and down. The lights don't get dim anymore but everytime I turn on something such as a/c, headlights, radio the volts will drop down. Right now with nothing on its barely over the 13 mark. And when I turn on the a/c it will be right on the 13. It used not be like this until that day.
Since then the car just don't run near as good. The car went from 250 miles per tank of gas to barely over 200 miles per tank. I even put a new voltage regulator but didn't help none. So what could the problem be. Coil and module is about month old and o2 sensors are about 2 months old. Can anyone please help me.

I would highly suggest getting your alternator tested. :)

What exactly does the test do. Does it just show how many volts it putting out or what. I did have it tested yesterday at Advance. There tester hooked to the battery and they said the alternator showed that it was good. They said if I took the alternator off and bring it back they could do it again with a different tester. Should I have it tested, I already ordered a new alternator and its supposed to be in tomorrow.