I have a 98 Saturn sw2 with a recent problem. Two times now it has stalled while idling and when it doesn't stall it floats near 500 rpms (Usually 750 - 950). The first time, it would start and just drop down to nothing, the next was coming to a stop. These times have been remedied by using the gas pedal to regulate the rpms and keep them in the normal range. It may be worth noting that the first time it happened was after filling up from near empty (emptier than I usually let it) and there was a bad storm the night before. Is it a sensor, some blockage or did I get bad gas?
Long shot, but do you have a voltmeter? Check the voltage at the battery when the car is running, might be a faulty alternator.
There's a chance you may have developed a vacuum leak, are you handy? You can do this yourself with a bottle of brake cleaner or throttle body cleaner. Spray around the engine bay (not coolant hoses, but vacuum specifically) and if you hit a spot where the revs go up, well, you've found you're vacuum leak.