The popular misconception is the diagnostic fault code did not say the IAC valve was bad or needed to be replaced. Fault codes never say to replace a part. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition.
As with most other systems, Ford does things differently than almost every other manufacturer. Their idle control system is no exception. Your valve is a two-wire spring-loaded solenoid. Problems related to carbon buildup in the air passage, or wiring problems, like corroded connector terminals, all result in the valve pulling closed or, when open, not allowing in enough air. All of those cause low idle speed. The only way for idle speed to be too high is for the Engine Computer to request it, or if the valve is sticking after being previously commanded to open. We can rule out the sticking valve since you replaced it already.
One thing you can start with is to unplug the IAC valve while the problem is occurring. Idle speed should drop real low. If it does not, suspect a vacuum leak.
The easiest way to approach this is to connect a scanner so you can see what the Engine Computer is responding to. If you see the computer is requesting a lower engine speed when actual speed is too high, it is not having success. That points to the vacuum leak. If you see it's requesting a high idle speed when actual speed IS too high, that is in response to something the computer is seeing. The most common cause of that is erratic coolant temperature sensor readings. Normally temperature sensors of any kind are extremely reliable because they have jut one component inside them, but that didn't prevent Ford from having a huge rash of failures in the early '90s. That was solved by '98, but the same symptoms are often caused on any brand of car from corroded sensor connector terminals or broken wires.
Monday, May 1st, 2017 AT 4:33 PM