Saturn - SL2 :: 1999 - Engine Rev Up To About 2000 RPMs While At A Stop Then Drop Down To About 650

My '99 Saturn SL2 has from time to time in the past year or so had brief bouts where the engine will suddenly rev up to about 2000rpms while at a stop and/or drop down to about 650rpms combined with surging and stalling while moving. The Service Engine light would typically come on in conjunction with these spells, but then a few days later, things would be back to normal and the light would go off. The episodes seem to correspond with weather that has high humidity, but not necessarily extreme heat. Once or twice when the heat was above 90, the car quit completely and wouldn't restart for several minutes, but that was last summer.

About a month ago the car, which is normally parked in a garage, spent the whole weekend outside when we had some extreme rains (in air temps only about 60s or so). Since that weekend, the car is running frequently with the issues previously described, especially the surging and stalling while driving. It will feel like the timing of the fuel is all out of whack, will shudder and run hard, and then it will suddenly correct itself and run normally. A friend has a code reader and the codes that came up are 404 and 1404, which indicate the EGR valve. We replaced that with a new one and put in 4 new spark plugs, but the problem still persists. I believe the fuel pump is in the gas tank, so from other things I've read, a vapor lock may not be the problem since the fuel is being pushed.