Engine is about to stall

I have a 97 Nissan Altima (automatic) and at the odd time when I am at a red light or at a complete stop idling, my rpm's drop from the proper 800rpms to between 400-500rpms and then jumps right back up. This doesn't happen all the time, maybe 3 to 4 times a week. I seems like the engine is gonna die or stall. My spark plugs are overdue. Can that be the problem?

Thanks in advance

same thing happens to my car.
But my car dies some times. it freaks me out.
Have you foud out why?

mine does the same thing too. I think it might be the fuel filter but I would love to get someone else's input on this.

Yup, same thing happens to my 97, but I havnt gotten around to fix it yet. It might be the throttle position sensor and a new fuel filter would help as well. It normally happens when you come to a stop and just like 95rsv said, idle rpms drop from 800 to 500 and below. It starts to sputter and 60% of the time it stalls. Whats crazy is that it will start back up in neutral and most of the time get going again when put in drive. It sucks when your the first one at a red light and theres people behind you and you have to go through all that ish. My question is, has this problem effected anyone's gas mileage? My gas mileage has been retarded for a while and Im trying to figure out whether it's because of this or something else. Im getting like 225 miles out of a full tank. Fuel capacity is 15.9 gallons and gas prices around here are $2.06/gal. The 97 Altima is suppose to get 21 city 29 highway. Its depressing.