I recently replaced my voltage reg and generator, and for some reason at less than 1000 rpm, i hear a click in the regulator and the light comes on, then sometimes won't go off until i hit about 2000...
not so much lately, but it was really bad when i first did it.
Maybe it's just corrosion on old wires, and you aren't getting a good ground.
That has also happened to me. I've had electrical problems, and sometimes just replaced/fixed a corroded terminal or something and helped it.