1994 Other Kia Models lights

I have a 1994 Kia Sephia and sometimes when i start the car and turn on the lights, the headlights and the dashboard are very dim but the car still runs but feels weak. Other times when i start the car, the headlights and the dashboard are bright and normal. Now here's something weird, sometimes when i'm driving the car while the lights are dim, all of a sudden while in mid drive, the lights get bright and i feel an increase in power.