1999 mustang gt - Stalling

Tsnyder124 said:

And also, my passenger side long tube header is rusted a little bit, with some holes. Maybe that could have the O2 sensors mis-reading and that could be the issue, not sure.

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^^^THIS would be my vote. The unmetered air in the exhaust will make the PCM "think" the motor is running lean. The PCM will respond by adding fuel. Since the AF mixture isn't really lean, the extra fuel will make the air fuel mixture RICH. Recommend fixing the outstanding DTC before replacing a ton of parts.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

After the first round of repairs, please post the full DTC codes. There are 1,000's of them and they each have a story to tell.

My dads truck which has the 4.6 liter motor was doing the exact same thing. He had an exhaust leak which was throwing the oxygen sensor off and it was adding extra fuel. It would idle up then down then stall. Fix your exhaust leaks before you do anything else otherwise your chasing your tail.