hesitation/stalling

I have a 91 Z24 and recently when I am driving it will run fine then when I stop for a stop sign or light the rpm's go up and down and then it stalls. It always starts again after about 20-25 seconds of cranking but the same thing might or might not happen at the next stop. I was told that it might be my crank position sensor because it was oily so I replaced that and it seemed od for a couple of days and then right back to the same problem again. I have noticed that there is a slight rattle from the cat, if it was plugged it would stall all the time. Has anyone had the same problem? :confused:

Thanks

sounds like IAC valve