1997 Pontiac Firebird breaking up, missing under load when

I have just entered a message regarding this car yesterday, and this is some follow up information. When the car is cold it starts right up, and shortly thereafter it starts to fluctuate in between 1500 and 2000 rpm's like it's trying to comensate for maybe an air / fuel mixture. So after doing this for a minute or two - the engine finally idles down to a steady 900 rpm. Also, in my other question I listed that once engine warms up after driven 15 - 20 miles it starts to break up, sputter, have an intermitent miss, but keeps going. This condition seems to happen when the engine is under a slight load like accelerating, or going up a grade. Any ideas - thanks - RAY