Help needed. My 360 idles rough and spits and pops a little while it is just starting into just below the operating range on the temp gauge. It runs fine after the temperature enters the operating range but if you shut it down for a half hour and then try to drive off it really runs bad for a while. The rough running correlates with a loud hissing from the throttle body from where the opening that feeds air via the IAC is. Whatever is commanding the IAC to fully open is probably causing the problem. This is where I don't have a clue as to what or where. IAC has been replaced, Plenum doesn't leak. Also no codes are set but sometimes a too rich code or O2 sensor code is.

Any suggestions will be appreciated.