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Is the check engine light on?
Does the vehicle try to lunge forward or reverse when this concern is happening?
Does it idle correct in park?
Hello I'm Chris a Ford Certified Transmission specialist.
Thanks for asking your question. I am eager to get your vehicle back up and running correct, I am here to help you out for a small fee far less than a dealership, so if this information is of aid PLEASE PRESS ACCEPT and help me out financially. Please reply if further discussion is needed.
You need a transmission tech to do some diagnostics with a scan tool. it sounds like the torque converter clutch is trying toengage either from low fluid or fluid pressure loss on the tcc circuit. You have already replaceed the IAC and I assume vacuum and PCV hoses were checked for leaks.