What are the symptoms of a bad transmission coolant regulator

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Right now I am in Las Vegas it's very hot, transmission won't shift RPM is at 3000 MPH 25 won't shift out. After the car cools down it runs perfect. A mechanic told me it was the transmission temperature control regulator. The part is a $100 and to install it is $100 would the transmission do that if it was the regulator...?