Ricardo,
Could be as simple as a loose or corroded wire, could be alternator going bad... Check belt tension, although that most likely isn't your problem (and it would go away at higher rpms). Start with the simple things (check wires on alt. and battery cables). If that doesn't help, have the alternator tested.
Sounds like it may be starting to go bad, and is putting out just enough voltage to "tickle" the light, but not turn it on.
Carey