low rpm=no battery light, high rpm=bright battery light

[Habanero]

A search of the archives turned up nothing similar to this problem, so here we go. Last night after getting groceries, I went out and started the car. Idleing through the parking lot, my battery indicator light came on very dimly. As soon as I accelerated on the street, the light became very bright. Throughout the trip home, it showed this same pattern. The brightness of the light was a direct correlation to the rpm of the engine. At highest rpms the other lights began to glow dimly as well. Always when it came back to idle, all the lights were off. I have often seen the reverse of this situation where higher rpms charged the system, but have never seen this situation. Any thoughts other than the usual brushes or regulator?

By the way, it is an 87 300SDL.