Check the voltage regulator and the ground to it and the alternator. I had a similar problem last week and it turned out that the new voltage regulator I bought was junk so I had to buy another one. To add to it, my grounds were not properly done. As some members on this forum have told me, grounds are one of the most important things to remember when dealing with electrical problems. Here is a link to my post and the resolution to my issues.
http://67-72chevytrucks.com/vboard/s...tage+regulator
Good Luck