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.
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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.