Cadillac Repair: 94 Sedan Deville high Idle, driveability problems, cadillac sedan deville

Question
my 1994 Cadillac Sedan Deville will sometimes hit a high idle (full throttle, 5000 RPM) I disconnected the IAC (Idle Air Controller) and it runs fine other then it won't stay running I replaced the IAC and it has the same problem. took it to a local GM Dealership and paid $300 to have them tell me they don't know what it is. PLEASE HELP !! I can't be the only case

Answer
Has there been any work of any kind what-so-ever performed on the vehicle in any manner at all. ANYTHING ?????.

IS or HAS the CHECK ENGINE LIGHT ever illuminated on the dash when driving the vehicle ?.

Did this problem just all of a sudden appear or were you experiencing any idle roughness or driveability problems prior to this high idle ?.

Are you POSTIVE that you have an IAC and not an ISC ?. In 1994 they did NOT utilize an IAC, They used an ISC.