Head Lights Turn off after 10min

I'm new to For A bodies only and I need some help. When I drive my car with the head lights on at night they will only stay on for about 10 min. or so I can switch the lights off for 30 to 40 sec and it will give me another 5 to 7 min. of headlight power. If some one can give me some pointers to help me fix my problem I would truly appreciate it.

(I have recently put a new alternator and regulator and the other day I figured maybe my battery was running low so i put it to charge, after 5 sec the green light came on saing my battery was at fully charge)