No codes set? Do you have live-data? If no other info is available, I would check fuel pressure (with a gauge!) and try blocking off the EGR valve, if it has one. Also, inspect all vacuum hoses, especially the ones for PCV and brake boost, for cracks that could be opening up and causing an intermittent vacuum leak.
Hmm, yeah need more info. No mention of what engine. There were 4 made for this year car. Vin numbers 3, 5, P, Z. Three with 2.0l's, two of which are DOHC, the other SOHC, the last one is a 2.3l, All are 4 naggers obviously.
Surging while driving is quite different then at idle. I would certainly check for air intake duct cracks. Surely will cause surging under load/driving and will usually settle down at idle as the crack folds into itself. Easy enough check to do. Just flex the intake air duck back and forth while at idle and notice idle changes or downright stalling.
And, as stated above in previous post, is live data available to you, if so, you can certainly post if there are any codes and post the fuel trims while it surges.