CHARGING SYSTEM INDICATOR GOES TO "RED ZONE"

'87 Bronco, A/T, new engine w/ 9,000 mi.

When the car drops down to a lower gear on the hwy or city, the charge indicator gauge hits the red zone and the battery warning light on the dash comes on (RED). When these two things happen, I notice that the dashboard lighting is brighter and the headlamps are brighter as well. When the trans goes up to a higher gear, the charge indicator gauge drops back to normal and the warning light ceases.

Electrical stuff? Wrong battery? Over-charging?

Please help!