2000 Buick Century Advice on what to to about this problem

My 2000 Buick Century has a service engine light on, I had it read at a local Auto Zone store, they said it was a bad EGR valve. I had it replaced. Approx., 3 days later the service engine light came on again, I had it read again at the same Auto Zone, they said it said that the EGR valve was reading insufficient. While driving the car it runs fine while cold, when the temperature gauge starts moving towards the hot zone, the car starts shifting very hard while changing gears, if I turn the car off and let it cool , it runs fine again and shifts smoothly. What could the problem be ? I appreciate you time and I await your answer.

Thank You, Mr. Billy Simpson III