I would say it's not the transmission!
Are there any Engine or Transmission codes?
I would connect a scanner and read the parameters of the transmission and engine to see what the sensor signals are doing.
Is he resetting the transmission by disconnecting the battery? Not a good idea if true!
If so, he is also resetting the engine computer, so you can't tell which is creating the problem. Make sure just the transmission computer is reset, or just the engine computer.
He may be a good mechanic, but you need to get someone else to look at this.
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