For your 1999 Pontiac Grand Prix there is a possibility that the torque converter clutch may be locking up, causing the engine to stall out. Or it could a transmission issue. Best of luck.
If you need help finding a technician, please find an AskPatty Certified Repair Shop at http://www.askpatty.com/search/?type=all&m=30&q=&keyword= and they can help you diagnose and repair the problem.
〉 Answered on Dec 12th, 2015 by Maryann Croce, Owner at Croce’s Transmission Specialists