Could be a problem with the EGR valve. Carbon deposits can and will get stuck in the valve and passages in the intake manifold. The passages can all be cleaned out using carb spray. When you reinstall the valve, use a gasket with a screen in it.
http://shop.advanceautoparts.com/web...S_271725256___ Usually if the valve doesn't close completely, the SES light will come on and set a DTC in memory, but if the passages are partially restricted, it could cause a surge without turning the light on.
Have the oxygen sensors been replaced yet? Oxygen sensor reaction time slows with age. At 100,000 miles they have slowed significantly. Possibly enough to cause a lean surge. A scan tool can spot this before the SES light even thinks about coming on
Might want to at least have them checked, if not replaced because they're due. There are 3 of them on your 98 and it's not a difficult job to replace them.