1999 Pontiac Grand Am 2.4 idles rough, stalls

Car will idle at 6 or 7 hundred rpm, then suddenly drop off to about 500, sometimes less. If it's in drive, sometimes this stalls it out. Also, when decelrating from a 55 zone to go into a 30 zone, I notice the tachometer pulsing rythmically until I'm down to 30. Then sometimes it feels like it surges at the same rhythm as before while trying to maintain 30mph. When idling at a stoplight, it misses occasionally and smells like a rich mixture. I tried a can of injector cleaner, which helped very slightly for the duration of that fill-up, but by the next fill-up, it was back to (ab)normal, maybe even a little worse. I sprayed around to check for vacuum leaks, but none were found. There also is a Car Repair Talk's forum.