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You are describing nearly a textbook issue with a bad injector. The converter isn unlocking because the engine is dropping torque which is causing the pressure in the transmission to go up and down which is what is causing the "popping" that you are referring to.
I would have the injectors tested and a compression test done with the Ford IDS tool and see which cylinder is showing the failure.
This is based on what you have described.
Now let's take this just another step here for just in case. If the the injectors do test good, then the torque converter itself is causing the issue most likely due to the solenoid. The solenoid may be opening all the way, but closing only partially causing the torque converter to "pop" and put stress on the output shaft and binding. I have seen this quite a bit on vehicles that tow a lot. Usually it is caused by a little piece of metal from transmission wear that gets caught in that solenoid and causes the torque converter to act up. The best way to really test it is to drive it, again with the Ford IDS hooked up, and monitoring the transmission output pressure and the torque converter lock/unlock commands. The tech can manually select to lock and unlock to replicate the issue and narrow it down. I usually find it to be the torque converter solenoid or the "A" solenoid for the main pressure off the pump.
If they do find a solenoid issue, have the fluid changed out as well as the filter so you are good for awhile. I would also have the cooler flushed to rid of any metal that is in the system. The metal isn't major. It is normal wear from a vehicle that tows. They tend to get metal flakes due to the extra strain on the parts and the little bit extra twisting on the gearing and bearings.
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