Yep. The 63-65 amp 10SI has a drop resistor in it that forces the alternator to "energize" at low RPMs to aid in charging under heavy loads at idle, and when started in case engine rpm arent fast enough to get the ball rolling.
Low output at slow enigne speed may keep the light out, but higher RPMs and regualtor demand will set it off as it cant produce enough current to keep up with the regulator demand.
Sounds like one of the 3 phases in the alternator is not outputting.
Fact is stranger than Fiction