What causes speedometer and tachometer to fluctuate at idle on 96 grand am and cause engine to rev up and down for some minutes then stall?

Do you have a check engine light? If so, get it scanned by a repair facility to make sure you don't do any unnecessary repairs. It could be many things, such as an o2 sensor or throttle position sensor just to name a couple. If you don't have. Check engine light, sounds like a faulty engine control unit. Again, have someone scan it with a computer. It will save you money in the long run.