92 Cadillac Seville idle problem

My 92 Seville has140,000 miles on it and when I got it the minimum air (idle speed) screw was missing but the car was running fine then 2 months down the road it started dying at low speeds in traffic or at red lights the idle fluctuated from high to low very quick I stopped driving it for one week and now it has a hard time starting up and idles at 500 Rpm for two seconds then dies out. I changed my spark plugs and wires gonna change the starter after July 4 and gonna change idle motor because has an idle screw I really don't know how to diagnose a vehicle but could use any information or ideas a person has to help cause I love that car

You need to set the minimum base idle using the set screw that you say was missing, then the idle control motor takes over, also the tps may need adjustment, this procedure is tricky I suggest getting a good shop manual.

Thank you so much