That light indicates a charging system problem. It kind of sounds like the belt is slipping at high load. How old is the belt? and tensioner? Also, if your batt is 4 years old it could very well be dying to.
Red '96 GT
PI heads and Intake, VT stage II's, Densecharger CAI, Pro-M 85mm MAF, Accufab 75mm TB, 1/2 " intake spacer, MSD Coil packs and wires, Mac Longtubes, Mac O/R H, Flowmasters, Steeda Tri-Ax, Spec Stage II clutch, AL flywheel, AL driveshaft, 3.73's, Max Speed Progressive rear springs, KYB AGX adjustable shocks/struts, GMS upper and lower rear control arms, UPR tubular K and coil overs, Cobra rear swaybar, full lenth subframes, Brembo cross-drilled and slotted rotors, high-temp Kevlar pads, poly bushings everywhere.