I have a 1999 4 wheel drive with Vortec 6000 V8 SFI gas engine. My problem is after I drive about 25 miles and have to stop at a traffic light or just stop the car wants to stall. Then repeatably each time I stop this happens. This problem did not exist until after my mechanic replaced the exhaust manifolds, spark plugs and spark plug wires. He claims everything is set to specs and when he runs tests everything checks out. He has also driven the car and has experienced the same problem of stalling. Any suggestions?