Idle Problem

Last year I had my low mileage1985 L98 into a local shop regarding an issue with the idle they said that the I.A.C. (idle air control) needed to be replaced which was done. Since then I've replaced plugs, wires, cap & rotor cleaned the throttle plate and had the injectors cleaned. Currently my idle is not stable it varies from 400rpm to 800 rpm, both the shop owner and the mechanic that worked on my car own C4 Vettes. They told me now that I may have a vacuum leak but another mechanic at a GM dealer said that I needed to get an OBD 1 scanner to set the relearn. Any help or suggestions would be greatly appreciated.