i had the same prob, hit 3500-4000 in park, car stumbled. ended up getting the light. took it in and had the intake runner controller(?) replaced under warranty. would have cost alot of $$$ if i didnt have warranty. but made a huge difference.

Black '98 Mystique V6 atx. superchip,2 1/4" SVT style dual exhaust (custom) with no cat, high flow res, flo-pro mufflers with Ractive tips, 8.5" K&N conical with CTA adapter, UR Ultra Street Pulley, TH-fix. 215/45/17 Bridgestone RE730's on Konig Absolute wheels. Trans go Shift Kit, High stall torgue convertor, 24000 BTU transmission cooler.