Car is surging and chugging

I have a 1994 nissan NXR coupe. It has done approximately 158,000km and I have it serviced every 10,000kms. I had a service 3 months ago, but the car has been surging and chugging ever since. It now surges when it is idling and chugs when I am driving around town. I took it to my local Nissan dealer and they spent 4hrs on it but were unable to find what was actually wrong with it. They agreed that there was a problem and indicated that they 'had put the car back together as best they could! I mean, really, the people who sell and fix these cars everyday were unable to find what was wrong with it. :( Does anybody have any suggestions?