Well that's the only time the EGR is used, under load.
But you would usually get pinging/knocking not mis-fires without Exhaust gas to cool the cylinders.
ICM(ignition control module) is in the Computer(PCM) on the '98, but I would think 2.5l or 2.3l wouldn't matter.
Is your idle at 700rpm with warm engine?
If higher you could have a vacuum leak which will lean out the fuel mix under load.