You must identify your year, engine, and transmission. Without this information, we are unable to provide other than generic information.
First, check for vacuum leaks.
If none, replace the IAC. You've been wasting your time and money replacing the other stuff when there was no reason to do so.
Upon replacement of the IAC, disconnect the battery so that the EEC is forced to relearn idle strategy.
There is no evidnece from the sypmtoms you listed of any fault in the fuel system. The 542 is common on a manual transmission if you stall the engine.
Steve
