1994 "hiccups"

Has anyone ever heard of a 1994 concorde "hiccuping"? It will be running fine, and this happens mostly in stop-and-go traffic (ie. rush hour) where I'll be travelling along at 50mph, then all of a sudden it sort-of hiccups and jumps for a quick second to maybe 52mph and back down again. Tranny's been checked and the shop didn't find anything. I'm not even sure where to go from here. Can anyone help?