Could be the coolant temp sensor (the one that feeds the ecu, not the gauge. The blue one iirc). After I fixed all my vacuum leaks the surging stopped but the car was running so rich it wouldn't warm start. The resistance of that sensor increases as temperature decreases; the sensor's internal resistance also increases with age and wear. So an aging sensor can be telling the ecu that the engine is a lot colder than it really is, hence running rich. When I got my car I think it was only running because the vast amount of unmetered air from the many vacuum leaks sort of balanced the vast amount of extra fuel from the dead CTS.