Most likely, the computer is allowing the engine to run rich before warm-up (normal) and this produces the power increase you experience.
You could try a 180 thermostat and see if it works.
Another option is installing an
adjustable fuel pressure regulator which can be set to deliver more fuel then the factory regulator.
Not certain about availability of those for the 3.8 but you could check JEG'S or SUMMIT.
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