If the engine has good power above 3000 rpm, then the converter is not plugged. Engines are, in simple terms, big air pumps. They draw air in, mix it with fuel, burn the mixture, and pump the results out. The faster the engine runs (rpm), the more air in and exhaust out. With a restriction in the exit, the symptoms would first show up when the engine was running fast and trying to expell exhaust in large volume.

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1989 300 SEL that mostly works, but needs TLC