Car looses power while the engine speed still perfect?

I changed the water pump in 1992 camry le 2.2 after it suffered a very small overheating. The service normalized cooling, but the car loose power while the engine speed still perfect. Could be the automatic transmission fluid that needs replacement or is a flush in order?

The water pump was replaced during a short trip, and the car never ran super overheated. During the service nothing else wrong was found, I replace also the spark plugs and the spark plugs harness and connectors are in good shape. I felt that the fluid color changed from dark red to almost brown, but I am not sure if the car lost power just because this.

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