Electrical Overcharging Issue.

Hello,

My 94 Mustang GT is overcharging by that I mean The voltage gauge pegs. It only starts to do this after about 20 minutes of driving and after I get on it. It has a rebuilt alternator that bench tests perfectly and a new voltage regulator as well. Battery is fine, tests fine,car starts right up and runs perfect for about 20 minutes, then it overcharges under load. I am stumped. Could bad pigtails that plug into the alternator cause this? Like I said this only happens after driving the car for about 20 minutes and after say I do a burnout or slam a couple gears.

Thanks, Corbin

Thanks for the response.
Well what concerns me is all my lights get brighter, and my air bag light came on? Would that happen if just my gauge is bad.