'99 Ford Tarus

The car does not stay idiling when I first start up in the morning. After approx. 20 miles, it starts idiling high. Someone suggested it my be the catalytic converter?

there is a ton of things that can cause I would stop by auto zone and they can help read your codes. I,am thinking on the line,s of a coolant
sensor not telling the cumputer the engine is cold and the idle is not in cold mode. the idle speed motor may be stuck . I will guess if you
push down on gas pedal just slightly the engne will idle . does this work?

you said it might be the cat when the catalytic converter go,s bad it slowly get clogged and what happen,s is a lose of power after the engine and the CAT gets hot.