2000 Buick Century

I have a 2000 buick century 140,000 miles on it. It runs great, but recently after I have been driving it aprrox 15-20 minutes and I get to a red light it starts to jump at idle. Its like for a quick second it seems like it wants to stall but it never does, and then when the light changes its fine....shifts smooth and engine seems strong. I took the vehicle to a repair shop but of course they could not duplicate the problem. I just had the wires and plugs changed so its not that. Anyone have any idea what it could be?? It only happens when its been running for a little while and Im at a light or stuck in traffic. Thanks