So I have a 2.8L/173cid 1986 firebird and have been having issues with my idle going anywhere between 950(current idle setting) to 2200. The most common things it does include but are not limited to; staying at 2000rpm, hovering just about 1000rpm, and bounce between 1400 and 1600 rpm. Rarely I've seen it; stay at 950 as intented, bounce above 2000 to 2200rpm, and stay at either 1700 or 1100. I've done some stuff as per my own knowledge and the knowledge of my fellow mechanic coworkers and foreman. These things include: blocking off remaining EGR system at the throttle body to eliminate possibility of intake air leak, throttle body service/rebuild, testing electronic idle control module, numerous leak tests for fuel and air, throttle cable and throttle plate return sping checks, and many misfire tests. Codes do come back sometimes but my car seems to delete the code before we have a real chance at looking at it from that angle. My last idea is the M.A.S but my colleges and I think its a far reach. Last thing to note is that my timing has been retarded for better high rpms performance but that was done long before I ever owned the car. Does anyone have any idea what could be going on here?