[coco1476]

Hi everyone!
I think i might have an alternator problem. When i'm driving at night, as soon as i push on the brakes or use the heater or the engine fan comes on or uses any kind of electrical power, i see my headlights and my dashlights starting to flash, they do not flash like a flasher but they go dimmer and bright again, and the engine starts to stumble, everytime the lights go dimme the engine looses power. I did test the alternator with a tester with the engine running as soon as i use electrical power of the car the meter range from 12 volts to 14 volts just like a clock 12 volts-14 volts-12 volts-14 volts. Does anyone know what could be the problem?
Thanks