As I stated in the PM to you, and others have stated:
The possible causes for YOUR scenario (light on, not all the time, under load more often than not), are:
Alternator starting to go out.
Serpentine belt slipping (not busted.. busted would be a no charge scenario)
That's it.
I wouldn't suspect the megafuse (it would be an all/nothing condition, like a broken belt) and I wouldn't suspect the bruses, yet. They normally float at upper rpms, not lower rpms with load.
My biggest culprit, so far, would be slipping serp. belt, however an alternator isn't out of the question, yet. (There are other parts to an alternator that CAN fail and be present at lower rpms, and brushes CAN float at lower rpms.. however unlikely it may be)
Ray
